1660s, "heinous, extremely criminal, enormously cruel," from stem of Latin atrox "fierce, savage, cruel" (see atrocity) + -ous. Weakened colloquial sense "very bad" is late 19c. Related: Atrociously; atrociousness.
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.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."