1530s, "enormous wickedness," from French atrocité or directly from Latin atrocitatem (nominative atrocitas) "cruelty, fierceness, harshness," noun of quality from atrox "fierce, cruel, frightful," from PIE *atro-ek-, from root *ater- "fire" + root *okw- "to see;" thus "of fiery or threatening appearance." The meaning "an atrocious deed" is from 1793.
one of the Dinosauria, a class of extinct Mesozoic reptiles often of enormous size, 1841, coined in Modern Latin by Sir Richard Owen, from Greek deinos "terrible" (see dire) + sauros "lizard" (see -saurus). Figurative sense of "person or institution not adapting to change" is from 1952. Related: Dinosaurian.