What is the X in X-mas?
Old English hwi, instrumental case (showing for what purpose or by what means) of hwæt (see what), from Proto-Germanic *khwi (cf. Old Saxon hwi, Old Norse hvi), from PIE *qwei, locative of *qwo- "who" (cf. Greek pei "where"). As an interjection of surprise or to call attention to a statement, recorded from 1510s.