What is the X in X-mas?
early 15c., "chance, accident; incidental charge," from casual (adj.) on model of royalty, penalty, etc. Casuality had some currency 16c.-17c. but is now obsolete. 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.