What is the X in X-mas?
late 14c., "barb of an arrow," from Old French barbe (11c.) "beard, beardlike appendage," from Latin barba "beard," perhaps cognate with Old English beard (see beard (n.)).
late 15c., "to clip, mow;" see barb (n.). Meaning "to fit or furnish with barbs" is from 1610s. Related: Barbed; barbing.