early 15c., casuelte, caswelte, "chance, accident; incidental charge," from casual (adj.) on the model of royalty, penalty, etc. From the earliest use especially of untoward events or misfortunes. The meaning "losses in numbers from a military or other troop" is from late 15c. The meaning "an individual killed, wounded, or lost in battle" is from 1844. Casuality had some currency 16c.-17c. in the sense "chance, a chance occurrence," especially an unfortunate one, but now is obsolete.
Proto-Indo-European root meaning "to fall."
It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."
1961, member of the U.S. anti-communist John Birch Society, which was founded 1958 and named for John Birch, U.S. Baptist missionary and Army Air Forces captain killed by Chinese Communists shortly after the end of World War II, who is considered the first American casualty of the Cold War.