What is the X in X-mas?
Old English bær "naked, uncovered, unclothed," from Proto-Germanic *bazaz (cf. German bar, Old Norse berr, Dutch baar), from PIE *bhosos (cf. Armenian bok "naked;" Old Church Slavonic bosu, Lithuanian basas "barefoot"). Meaning "sheer, absolute" (c.1200) is from the notion of "complete in itself."
Old English barian, from bare (adj.). Related: Bared; baring.