In etymologies, thermodynamics (etymology) has an intricate history, spanning over forty-years (1824-1868) of naming subtleties, starting with French physicist Sadi Carnot's mention that a mechanical theory based type of heat engine theory was needed (1824), followed by about twenty-five years of discussion of variants on the term "Carnot's theory", to Scottish physicist William Thomson's adding together of the terms "thermo-dynamic" (1849) + "dynamical theory of heat" (1851) to mean "thermo-dynamics" (1854), variations on the German name of the subject, introduced by Rudolf Clausius, of "mechanischen wärmetheorie", translated into English, during the years of circa 1850 to 1878, as "mechanical theory of heat", to the removal of the conjunction bracket, meaning that the subject had essentially solidified into a commonly accepted term, "thermodynamics" in 1868, by various authors, such as Gustave Hirn and Peter Tait. The year in which representatives of the German school of thermodynamics, such as, e.g., Herman Helmholtz in 1882, began to adopt the English variant (thermodynamics) over the German variant (mechanischen wärmetheorie), can loosely be said to have been the final chapter in the history of the etymology of the term thermodynamics. The following table gives a chronological overview of the basic etymology:
In short, over the course of nearly a decade, William Thomson (in coordination with William Rankine and Rudolf Clausius) synthesized the subjects of Carnot's motive power of heat (1824), Joule's mechanical equivalent of heat (1842), and Clausius' mechanical theory of heat (1850), among others, into the subject known known as thermodynamics. A near-synonymous term used is "energetics", used early on by used by Scottish physicist William Rankine, and in later years by the energetics school. The joint term 'thermodynamics', as a solidified subject, began to be used as early as 1868, such as found in the works of those as Gustave Hirn and Peter Tait.
Thomson | 1849
The term thermo-dynamic was first used in 1849 by Irish physicist William Thomson (Lord Kelvin), in a paper titled paper An Account of Carnot's Theory of the Motive Power of Heat, where he states: 
“A perfect thermo-dynamic engine of any kind, is a machine by means of which the greatest possible amount of mechanical effect can be obtained from a given thermal agency; and, therefore, if in any manner we can construct or imagine a perfect engine which may be applied for the transference of a given quantity of heat from a body at any given temperature, to another body, at a lower given temperature, and if we can evaluate the mechanical effect thus obtained, we shall be able to answer the question at present under consideration, and so to complete the theory of the motive power of heat.”
— William Thomson (1849), “An Account of Carnot’s Theory of the Motive Power of Heat”, Jan 2; in: Mathematical and Physical Papers, Volume One (pg. 118)
Thomson | 1854
In 1854, coined the term 'thermo-dynamics' as a specific subject or branch of science. Specifically, influenced to an extent by the 1837 book Theory of Heat by English mathematician Philip Kelland, which used phrases such as a needed "theory of heat" in relation to "forces", and terms, such as the "dynamical hypothesis" of heat, etc., Thomson presented the following definition: 
“Thermo-dynamics: the subjects [of] the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.”
The majority of references cite this year (1854) as being the first time the word thermodynamics was used. A few other sources cite that Thomson used the term thermo-dynamics in 1850 or 1852, but there does not seem to exist an actual reference to corroborate these various 1850-1853 claims. 
Rankine | 1855
In 1855, Scottish mathematical physicist William Rankine, an associate of Thomson, in a paper titled “Outlines of the Science of Energetics”, had begun to differentiate between a “science of energetics”, loosely defined by his hypothesis of molecular vortices, a general law of transformation between actual energy and potential energy, and parts of Thomson’s theory of thermo-electricity, among other principles, and a “science of thermo-dynamics”, having to do with equilibrium of energies between bodies, absolute temperature, and Carnot’s function, among discussions. 
Rankine | 1859
In 1859, Rankine, according to the retrospect views of Scottish physicist James Maxwell, wrote the world's first chapter on thermodynamics titled “Principles of Thermodynamics”, in his 1859 book A Manual of the Steam Engine and Other Prime Movers.  In the opening section of this chapter, in reference to the results of the mechanical equivalent of heat, Rankine defines thermodynamics as such:
“It is a matter of ordinary observation, that heat, by expanding bodies, is a source of mechanical energy; and conversely, that mechanical energy, being expended either in compressing bodies, or in friction, is a source of heat. The reduction of the laws according to which such phenomena take place, to a physical theory, or connected system of principles, constitutes what is called the science of thermodynamics.”
Nichol | 1860
The first encyclopedia definition of 'thermo-dynamics' is give by Scottish astronomer John Nichol in his 1860 Cyclopedia of the Physical Sciences, wherein thermo-dynamics was defined, in short, as “heat or the theory of the mechanical action of heat”. More definitively, Nichol's defined thermo-dynamics as science “still in progress” and gave the following definition: 
“Thermo-dynamics: the reduction of the laws according to which the phenomena by which heat, by expanding bodies, is a source of motive power and, conversely, that motive power, being expended either in compressing bodies or in producing friction, is a source of heat, take place, to a physical theory or connected system of principles.”
Clausius | 1865
In 1865, German physicist Rudolf Clausius published his collected nine memoirs (1850-1865) on the subject of what he called "mechanischen wärmetheorie", which translates as either "mechanical heat theory" or "mechanical theory of heat" into the form of a textbook on the subject of thermodynamics, as it is definitively know today. Subsequently, the term 'mechanischen wärmetheorie' is that which Clausius preferred as the name of the subject he largely created. In other instances, to note, he also seems to have used the term "mechanischen theorie der wärme"; possibly this was an earlier pre-truncation version of the latter.
Clausius, to clarify, never used the word thermo-dynamics either editions (1865 and 1875) of his textbook Mechanical Theory of Heat. Clausius did, however, use the term "thermo-dynamic engine" (three times) as well as "thermo-dynamic machines" (once), in his fifth memoir (1856), as in reference any type of heat engine that works in a cycle, in what seems to be modeled on the earlier Thomson phrasing.
A specific source needs to be tracked down to determine in what specific year the conjugational dash was first removed in going from 'thermo-dynamics' (a new term) to 'thermodynamics' (an established term). As a starting point, we note that Scottish physicist Peter Tait used the term thermodynamics in the title of his 1868 historical overview book Sketch of Thermodynamics. 
See main: Thermodynamics (definitions)In the 1892 Lockwood’s Dictionary of Terms used in the Practice of Mechanical Engineering, the newly evolving term was defined as: "thermo-dynamics: the study which treats of heat as a form of energy or mode of work."  The term “thermo-dynamics” (with the conjunction) was used well into the 1920s, before it became commonly known as one word. It is likely that the 1923 textbook Thermodynamics, by Americans Gilbert Lewis and Merle Randall, being the most cited thermodynamics book of all time, led to this common name usage.
Other non-sensical etymologies
Somewhat paradoxically, French thermodynamicist Pierre Perrot, author of the excellent 1998 dictionary the A to Z of Thermodynamics, maintains that the term "thermodynamics" was coined by English physicist James Joule in 1858 to designate the science of relations between heat and power.  This supposition, however, seems dubious. In the collection of scientific papers, Joule never uses the phrase “thermodynamics”, but does use the term “perfect thermo-dynamic engine” in reference to Thomson’s 1849 phraseology. 
American biophysicist Donald Haynie claims that the word thermodynamics was coined in 1840, from the Greek roots therme, heat, and dynamis, power.  This, however, seems doubtful. A third illogical source claims that French mathematical physicist Henri Poincaré, who was born in 1854, and later published the 1892 French book Thermodynamique, coined the term thermodynamiques to refer to the new insights that developed from the first and second law. 
In tribute to poor science, Ukraine physical chemist Vitaly Prisyazhniuk, in his 2007 Encyclopedia of Earth article “History of thermodynamics” (an article reviewed by American mechanical engineer and thermodynamics professor Tom Lawrence), states “it was G. Black (1770) who was the first to use the term ‘thermodynamics’”.  Here, Prisyazhniuk not only typos G. Black for Joseph Black, but also seems to be un-aware that Black was a calorist, i.e. a proponent of the caloric theory and promoter of Antoine Lavoiser's caloric theory of heat, and thus at odds with Clausius' mechanical theory of heat.
The central founder of the entire subject, German physicist Rudolf Clausius, curiously, never seems to have used the term “thermodynamics” to designate the subject he created more to perfection than any other, but seemed to prefer the German phrase: "mechanische Theorie der Wärme" (mechanical theory of heat) or "mechanische Wärmetheorie" (mechanical heat theory) over the German "Thermodynamik". He never used the word thermo-dynamics either editions (1865 and 1875) of his textbook Mechanical Theory of Heat. Clausius did, to note, use the term "thermo-dynamic engine" (three times) or "thermo-dynamic machines" (once), in his fifth memoir (1856), as in reference any type of heat engine that works in a cycle, in what seems to be modeled on the earlier Thomson phrasing.
● human thermo-dynamics
1. (a) Thomson, William. (1854). Thermo-electric Currents, Preliminary 97-101, “Fundamental Principles of General Thermo-dynamics Recapitualted, (pg. 232). Transactions of the Royal Society of Edinburgh, vol. xxi, part I.; read May.
(b) Kelland, Philip. (1837). Theory of Heat, (pg. iv, pg. 5). Cambridge.
2. (a) Kelvin, William T. (1849) "An Account of Carnot's Theory of the Motive Power of Heat - with Numerical Results Deduced from Regnault's Experiments on Steam." Transactions of the Edinburg Royal Society, XVI. January 2.
(b) in Mathematical and Physical Papers (1882-1911), Vol. I, 119.
(c) Smith, Crosbie. (1998). The Science of Energy - a Cultural History of Energy Physics in Victorian Britain, (pg. 93: "here Thomson used the term 'thermo-dynamic' for the first time). Chicago: The University of Chicago Press.
3. Perrot, Pierre (1998). A to Z of Thermodynamics, (pg. 301). Oxford University Press.
4. Nichol, John Pringle. (1860). A Cyclopedia of the Physical Sciences: Comprising Acoustics, Astronomy, Dynamics, Electricity, Heat, Hydrodynamics, Magnetism, Philosophy of Mathematics, Meteorology, Optics, Pneumatics, Statics, and etc. (pgs. 411-427). London: Richard Griffin and Co.
5. Horner, Joseph G. (1892). Lockwood’s Dictionary of Terms Used in the Practice of Mechanical Engineering, (pg. 372). Crosby, Lockwood and Son.
6. Haynie, Donald. (2007). Biological Thermodynamics, (pg. 26). Cambridge University Press.
7. Balluffi, Robert W., Allen, Samuel M., Carter, Craig W., Kemper, Rachel A. (2005). Kinetics of Materials, (pg. 2). Wiley-Interscience.
9. (a) Rankine, William. (1859). A Manual of the Steam Engine and Other Prime Movers, (chapter III: “Principles of Thermodynamics”, pgs. 299-478). London: Charles Griffin & Co.
(b) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
10. Joule, James Prescott, editors: William Scoresby and Lyon Playfair, (1884). The Scientific Papers of James Prescott Joule. Great Britain: Physical Society.
11. (a) 1854 Thomson sources:
Bolton, John. (2000). Classical Physics of Matter, (Quotes: 1854, pg. 106). CRC Press.
Thermodynamics (1854): theory of relationship between heat and mechanical energy, from adj. thermodynamic (1849), from thermo + dynamic. Merriam-Webster Collegiate Dictionary (2000).(b) 1850, 1852 Thomson sources:
Thermodynamics (1854) - Online Etymology Dictionary.
Berger, Stefan. (2006). A Companion to Nineteenth-century Europe, (Quotes: 1854, pg. 342). Blackwell Pub.
Cengel, Yungus A., Turner, Robert H. (2004). Fundamentals of Thermal-Fluid Sciences (Quotes: 1852, pg. 32). McGraw-Hill. 12. Rankine, William. (1855). “Outlines of the Science of Energetics”, Read before the Philosophical Society of Glasgow on May 2, and published in the Proceedings of that Society, Vol. III., No. VI.
Sebastian, Anton. (2001). A Dictionary of the History of Science (Quotes: 1850, pg. 331). Informa Health Care.
13. Prisyazhniuk, Vitaly. (2007). “History of thermodynamics”, Encyclopedia of Earth.
14. Tait, Peter G. (1868). Sketch of Thermodynamics. Edmonston and Douglas.