Meaning

noun someone injured or killed or captured or missing in a military engagement

noun someone injured or killed in an accident

noun an accident that causes someone to die

originFrom casual, from Middle French casuel, from Medieval Latin casualitas and Late Latin cāsuālis (“happening by chance”), from Latin cāsus (“event”) (English case), from cadere (“to fall”). Originally meaning “a chance event” (compare casual, as in “casual encounter”), it developed a negative meaning as “an unfortunate event”, especially the loss of a person.

Etymology from Wiktionary, CC BY-SA 3.0