1530s, "enormous wickedness," from Middle 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.
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