In the history of science, thermo-dynamics (with a conjunctional dash) was one of the original terms used to define the newly developing science of the Carnot's motive power of heat (1824), Joule's mechanical equivalent of heat (1842), and Clausius' mechanical theory of heat (1850). The subject went by other names such as the "dynamical theory of heat" (a phrase used by William Thomson) or energetics (a term used by William Rankine) and was later called "thermodynamics".

Etymology: Thomson
The term thermo-dynamic was first used in 1849 by English 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: [2]

"A perfect thermo-dynamic engine is such that, whatever amount of mechanical effect which it might produce? Nothing can be lost in the operations of nature—no energy can be destroyed."

In 1854, influenced to an extent by phrases, such as a needed "theory of heat" in relation to "forces", and terms, such as the "dynamical hypothesis" of heat, in English mathematican Philip Kelland's 1837 Theory of Heat, Thomson had come to make the following definition: [8]

"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 fully defined; while a few other sources cite Thomson in 1850 or 1852. [11]

Etymology: Rankine
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. [12] Four years later, in 1859, Rankine, according to 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. [9] 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.”

Other definitions
In the 1860 Cyclopedia of the Physical Sciences, thermo-dynamics was defined in short as “heat or the theory of the mechanical action of heat” or in detail as a science “still in progress” defined fully as:

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. [4]

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." [5] 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/mini-encylopedia 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. [3] 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. [10]

American biophysicist Donald Haynie claims that the word thermodynamics was coined in 1840, from the Greek roots therme, heat, and dynamis, power. [6] 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. [7]

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’”. [13] 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.

Other notes
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: "Mechanischen Theorie der Wärme" (mechanical theory of heat) or "der Mechanischen Wärmetheorie" over the German "Thermodynamik". He never used the word in both editions (1865 and 1875) of his textbook Mechanical Theory of Heat. He did, to note, begin to 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.

See also
● θ∆ics
● human thermo-dynamics

References
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.
8. (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.
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).
Thermodynamics (1854) - Online Etymology Dictionary.
Berger, Stefan. (2006). A Companion to Nineteenth-century Europe, (Quotes: 1854, pg. 342). Blackwell Pub.

(b) 1850, 1852 Thomson sources:

Cengel, Yungus A., Turner, Robert H. (2004). Fundamentals of Thermal-Fluid Sciences, (Quotes: 1852, pg. 32). McGraw-Hill.
Sebastian, Anton. (2001). A Dictionary of the History of Science, (Quotes: 1850, pg. 331). Informa Health Care.

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.
13. Prisyazhniuk, Vitaly. (2007). “History of thermodynamics”, Encyclopedia of Earth.