early 15c., casuelte, caswelte, "chance, accident; incidental charge," from casual (adj.) on model of royalty, penalty, etc. From the earliest use especially of untoward events or misfortunes. Meaning "losses in numbers from a military or other troop" is from late 15c. 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 in the sense "chance, a chance occurrence," especially an unfortunate one, but now is obsolete.