What is the X in X-mas?
1818, from Middle English crist(s)-crosse "Christ's cross" (late 15c.), earlier cros-kryst (late 14c.), "referring to the mark of a cross formerly written before the alphabet in hornbooks. The mark itself stood for the phrase Christ-cross me speed ('May Christ's cross give me success'), a formula said before reciting the alphabet" [Barnhart]. Used today without awareness of origin. As an adjective, 1846; as a noun, 1848.