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callous (adj.)[callous 词源字典]: c. 1400, "hardened," in the physical sense, from Latin callosus "thick-skinned," from callus, callum "hard skin" (see callus). The figurative sense of "unfeeling" appeared in English by 1670s. Related: Callously; callousness.[callous etymology, callous origin, 英语词源]
pachyderm (n.): 1838, from French pachyderme (c. 1600), adopted as a biological term 1797 by French naturalist Georges Léopole Chrétien Frédéric Dagobert, Baron Cuvier (1769-1832), from Greek pachydermos "thick-skinned," from pachys "thick, large, massive," from PIE *bhengh- "thick, fat" (cognates: Sanskrit bahu- "much, numerous" Avestan bazah- "height, depth," Hittite pankush "large," Old Norse bingr "heap," Old High German bungo "a bulb," Lithuanian biess "thick") + derma "skin" (see derma).
thick (adj.): Old English þicce "dense, viscous, solid, stiff; numerous, abundant; deep," also as an adverb, "thickly, closely, often, frequently," from Proto-Germanic *thiku- (cognates: Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (cognates: Gaelic tiugh). Secondary Old English sense of "close together" is preserved in thickset and proverbial phrase thick as thieves (1833). Meaning "stupid" is first recorded 1590s. Related: Thickly.

As a noun, "the thick part" (of anything), from mid-13c. Phrase through thick and thin, indicating rough or smooth going, hence "unwaveringly," is in Chaucer (late 14c.); thick-skinned is attested from 1540s; in figurative sense from c. 1600. To be in the thick of some action, etc., "to be at the most intense moment" is from 1680s, from a Middle English noun sense.
Pachydermata: "Originally, in Cuvier's system of classification: an order of mammals comprising elephants and related animals, and hoofed quadrupeds that do not chew the cud such as rhinoceroses, hippopotamuses, pigs, horses, etc. (now historical ). In later use (treated as plural, in form pachydermata): animals belonging to this order; pachyderms", Early 19th century; earliest use found in William Buckland (1784–1856), geologist and dean of Westminster. From scientific Latin Pachydermata, order name from ancient Greek παχύδερμος thick-skinned (Aristotle), after French pachyderme.