What is the X in X-mas?
c.1300, clete "wedge," from Old English *cleat "a lump," from West Germanic *klaut "firm lump" (cf. Middle Low German klot, klute, Middle Dutch cloot, Dutch kloot, Old High German kloz, German kloß "clod, dumpling"). In Middle English, a wedge of wood bolted to a spar, etc., to keep it from slipping (late 14c.). Meaning "thin metal plate for shoes, etc." is c.1825.