Pachydermos in Greek means literally "having thick skin" (figuratively, it means "dull" or "stupid"). It's from pachys, meaning "thick," and derma, meaning "skin." In the late 1700s, the French naturalist Georges Cuvier adapted the Greek term as pachyderme for any one of a whole assemblage of hoofed animals having thickish skin: elephants, hippopotamuses, rhinoceroses, tapirs, horses, pigs, and more. English speakers learned the word from French in the early 1800s. The adjective pachydermatous means "of or relating to the pachyderms" or "thickened" (referring to skin). Not too surprisingly, it also means "callous" or "insensitive" (somewhat unfairly especially to elephants, which are actually known to be rather sensitive).