Letter to Peter Guthrie Tait (13 Oct 1876)
An 1876 letter from James Maxwell to Peter Tait showing the "θΔcs" notation.
In thermodynamics, “θ∆ics” (also: θΔ or θΔcs) is the shorthand notation for 'thermo-dynamics' used in the 1870s correspondences between James Maxwell, William Thomson, and Peter Tait. [1]

Forbes
In c.1825, James Forbes, Scottish physicist, mentor to Scottish engineer William Rankine, at the Edinburgh University, the latter of whom being connected to William Thomson, contributed papers to the Edinburgh Philosophical Journal anonymously under the signature "Δ", likely in being a pseudonym for a person who is interested in heat. [8]

It could be possible, therefore, to conjecture, that the later Maxwell-Tait and Thomson circle usage of Δ in θΔcs, as Greek shorthand code for thermodynamics, was influenced or inspired by Forbes' age 16 pseudonym signature.

Thomson | Maxwell | Tait
In the 1870s correspondences between James Maxwell, William Thomson, and Peter Tait, Thomson, in his his letters, in particular, used phrases such as “2nd law of θΔ” or “2nd law of θΔcs”, among others. [2] The three of them likely used the symbol as a combination sound representation of thermo-dynam-ics, i.e. "theta-delta-ics", in association with the symbol delta "∆" having had a long history in science as the symbol of heat or fire. [3]

“I return the last page of Clausius. I have got the whole volume from the author. When you wrote the Sketch [of Thermodynamics, 1868] your knowledge of Clausius was somewhat defective. Mine is still, though I have spent much labor upon him and have occasionally been rewarded, e.g. earlier papers on molecular sorting, electrolysis, entropy, and concentration of rays. N.B. In the latter paper, reprinted in the volume, the name of Hamilton does not occur. When you are a-trouncing, trounce him for that. Only perhaps Kirchhoff ignored Hamilton first and Clausius followed him unwittingly not being a constant reader of the R.I.A. transactions and knowing nothing of H except (lately) his Princip, which he and others try to degrade into the 2nd Law of ΘΔ as if any pure dynamical statement would submit to such an indignity. With respect to your citation of Thomson, it would need to be more explicit.”
— James Maxell (1876), “Letter to Peter Tait”, Oct 13

In the early phases of the development of the science of thermodynamics, from the 1840s into the early 20th century, aside from θ∆ics, thermodynamics went by such names as "energetics", "mechanical theory of heat", or "theory of heat" among others.

The idea to use coded Greek letters to represent the term ‘thermo-dynamics’ (coined by Thomson in 1854), likely arose from Maxwell. During his early school years, at the Edinburg Academy at the age of 10, Maxwell frequently wrote to his father in zany letters filled with puns and misspellings, embellished with elaborate doodles, and containing secret messages in different colored inks. [5]

In the adjacent 1876 letter (or postcard) to Peter Tait wherein Maxwell uses, according to the 2002 editing of physics historian Peter Harman, the term “2nd law of ΘΔ” as shorthand for second law of thermodynamics.

According to the 2003 biography of Maxwell by English physicist Basil Mahon, however, its is stated that Maxwell used “ΘΔics”, rather than “ΘΔ” alone. [1] A third references it he Sklog wiki, discussed below, wherein it is indicated that “ΘΔcs” was used as the shorthand term. More research needs to be done to find a more definitive answer to this issue.

Macrakis
In 1997, Michael Macrakis, founder of the Greek Font Society (Ѻ), was titling his thermodynamics chapters as follows:

Thermodynamics (Macrakis)

using the Thomson-Maxwell-Tait Greek code shorthand for the subject of thermodynamics (see: θ∆ics), before it was called thermodynamics. [7]

Sklog wiki
The symbol \left. \right. \Theta \Delta^{cs} is used in the logo of the SklogWiki (in conjuction with S = K ln W) as shown below: [4]

SklogWiki

Human thermodynamics
In circa 2010, Libb Thims was testing the extrapolation, in respect to the science of human thermodynamics, knowing that Mars "♂" and Venus "♀" symbol symbolically represent man and woman, respectively, and knowing that the triple parallel line symbol "≡" is a representation of bonding between molecules, such as between human molecules, then the symbolic representation shorthand for human thermodynamics may be: Human (♂≡♀) Thermodynamics (θΔics) or "♂≡♀ θΔics".
In Thermodynamics We Trust new2
Above: The "ΘΔics" symbol, found at the bottom of every Hmolpedia article (linking to this page), being shorthand for the term "thermodynamics" (the science that governs the known universe), shown on a US one dollar bill, meaning, for the modern physical scientist, "In Thermodynamics We Trust"; substituted for original 1956 statement "In God We Trust" (a defunct theory), as adhered to by the general public. In 2010, Christopher Redford did a video (V) on God-reform in US currency and constitution wording.

IoHT
In circa 2008, in modern use, θ∆ics is the symbol for the Institute of Human Thermodynamics and is used as the bottom page icon for the Encyclopedia of Human Thermodynamics.

References
1. Mahon, Basil. (2003). The Man Who Changed Everything - the Life and Science of James Clerk Maxwell (pg. 132). UK: Wiley.
2. Maxwell, James. (1876). “Letter to Peter Guthrie Tait”, Oct. 13.
3. Opsopaus, John. (1998). “The Ancient Greek Esoteric Doctrine of the Elements: Fire”, University of Tennessee.
4. SklogWiki (About) - a Statistical Mechanics and Thermodynamics Wiki.
5. Lindley, David. (2001). Boltzmann’s Atom: the Great Debate that Launched a Revolution in Physics (pg. 79). The Free Press.
6. In God We Trust – Wikipedia.
7. Macrakis, Michael S. (1997). Scarcity’s Ways: the Origins of Capital: a Critical Essay on Thermodynamics, Statistical Mechanics and Economics (Boston Studies in the Philosophy of Science) (§:4: Thermodynamics (ΘΔcs), pgs. 93-). Springer.
8. Smith, Crosbie. (1998). The Science of Energy: a Cultural History of Energy Physics in Victorian Britain (pgs. 102-07). University of Chicago Press.

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