In famous thermodynamics quotations, entropy quotes are noted statements on the nature of German physicist Rudolf Clausius’ 1865 conception of the mathematical/physical term entropy. These quotes are listed below:

List

"If for the entire universe we conceive the same magnitude to be determined, consistently and with due regard to all circumstances, which for a single body I have called entropy, and if at the same time we introduce the other and simpler conception of energy, we may express in the following manner the fundamental laws of the universe which correspond to the two fundamental theorems of the mechanical theory of heat: (a) The energy of the universe is constant, (b) The entropy of the universe tends to a maximum."

— Rudolf Clausius, Mechanical Theory of Heat (1865) [1]

"Any method involving the notion of entropy, the very existence of which depends on the second law of thermodynamics, will doubtless seem to many far-fetched, and may repel beginners as obscure and difficult of comprehension."

— Willard Gibbs, Graphical Methods in the Thermodynamics of Fluids (1873) [2]

“By the introduction of the expression, “without compensation” (verses “of itself”), combined with a full interpretation of this phrase, the statement of the second principle (“that heat cannot without compensation pass from a colder to a warmer body”) becomes complete and exact; but in order to understand it we must have a previous knowledge of the theory of transformation-equivalents, or in other words entropy, and it is to be feared that we shall have to be taught thermodynamics for several generations before we can expect beginners to receive as axiomatic the theory of entropy.”

— James Maxwell, "Tait's 'Thermodynamics" (1878) [3]

"In those days I was essentially the only theoretical physicist there, whence things were not so easy for me, because I started mentioning entropy, but this was not quite fashionable, since it was regarded as a mathematical spook."

— Max Planck, "Commentary on joining (1889) the local Physical Society, University of Berlin" (c. 1940s) [4]

“As a young man I tried to read thermodynamics, but I always came up against entropy as a brick wall that stopped my further progress. If found the ordinary mathematical explanation, of course, but no sort of physical idea underlying it. No author seemed even to try to give any physical idea. Having in those days great respect for textbooks, I concluded that the physical meaning must be so obvious that it needs no explanation, and that I was especially stupid on the particular subject.”

— James Swinburne, Entropy: or Thermodynamics from an Engineer’s Standpoint and the Reversibility of Thermodynamics (1904)

“Entropy is a shadowy kind of concept, difficult to grasp … but again, we may point out that, the reader who would extend the notion of mechanism into life simply must grasp it.”

— James Johnstone, The Philosophy of Biology (1914) [5]

“You should call it entropy, because nobody knows what entropy really is, so in a debate you will always have the advantage.”

— Suggestion of John Neumann to Claude Shannon on what to call information (c. 1940s) [6]

“There is no concept in the whole field of physics which is more difficult to understand than is the concept of entropy, nor is there one which is more fundamental.”

– Francis Sears, Principles of Physics I: Mechanics, Heat, and Sound (1950) [8]

“Few physical concepts have caused as much confusion and misunderstanding as has that of entropy.”

— James Lovelock, Gaia: a New Look at Life on Earth (1979) [7]

“If we will be more forthcoming with explanations of our cherished terms, our science colleagues may be more inclined to help us with ‘entropy’, which to me is a more difficult concept than anything economics has to offer.”

— Tjalling Koopmans (1979) [10]

Undated

“The minute those two little particles inside a woman's womb have joined together, billions of decisions have been made. A thing like that has to come from entropy.”

– Rex Stout, American crime writer [11]

“Just as the constant increase of entropy is the basic law of the universe, so it is the basic law of life to be ever more highly structured and to struggle against entropy.”

– Vaclav Havel, Czech playwright [11]

In summation of these quotes, we may surmise that, according to Gibbs, Maxwell, Planck, Johnstone, and Neumann, entropy is to many a far-fetched, obscure, difficult of comprehension and difficult to grasp, mathematical spook, shadowy kind of concept, repulsive to beginners, which nobody understands, that requires several generations of teaching to become axiomatic or self-evident as a variable.

References
1. (a) Clausius, Rudolf. (1865). “On Several Convenient Forms of the Fundamental Equations of the Mechanical Theory of Heat”, Read at the Philosophical Society of Zurich on the 24th of April, 1865, published in the Vierteljahrsschrift of this society, Bd. x. S. 1.; Pogg. Ann. July, 1865, Bd. cxxv. S. 353; Journ. de Liouville, 2e ser. t. x. p. 361.
(b) Clausius, Rudolf. (1867). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies, (pg. 365), (URL). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
2. Gibbs, J. Willard. (1873). "Graphical Methods in the Thermodynamics of Fluids", Transactions of the Connecticut Academy, I. pp. 309-342, April-May.
3. (a) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
(b) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (II)”, (pgs. 278-81). Nature, Feb. 07.
4. [citation needed]
5. Johnstone, James. (1914). The Philosophy of Biology, (pg. 54). Cambridge: University Press.
6. (a) M. Tribus, E.C. McIrvine. (1971). “Energy and information”, Scientific American, 224 (September).
(b) Jeremy Campbell (1982): “Shannon does not remember von Neumann giving him such advice. However, Myron Tribus has a clear recollection of hearing Shannon tell him this story during a conversation in Shannon’s office at MIT on April, 1961.”
(c) See: Information theory.
7. Lovelock, James. (1979). Gaia: a New Look at Life on Earth (Introduction, pg. 2). Oxford University Press.
8. (a) Sears, Francis W. (1950). Principles of Physics I: Mechanics, Heat, and Sound (pg. 459). Addison-Wesley.
(b) Hokikian, Jack. (2002). The Science of Disorder: Understanding the Complexity, Uncertainty, and Pollution in Our World (pg. 27). Los Feliz Publishing.
9. Swinburne, James. (1904). Entropy: or Thermodynamics from an Engineer’s Standpoint and the Reversibility of Thermodynamics (section: Personal, pgs. 3-4). Westminster: Archibald Constable & Co.
10. Faber, Malte M., Niemes, Horst, and Stephan, Gunter. (2005). Entropy, Environment, and Resources: an Essay in Physico-economics (ch. 3: The Notion of Entropy, quote pg. 77). Springer.
11. Entropy quotes – BrainyQuotes.com.