What is the X in X-mas?
mid-14c., from a Scandinavian source (cf. Old Norse skamt, neuter of skammr "short, brief"), from Proto-Germanic *skamma- (cf. Old English scamm "short," Old High German skemmen "to shorten"), perhaps ultimately "hornless," from PIE *kem- (see hind (n.)). Also in Middle English as a noun, "scant supply, scarcity," from Old Norse. As a verb and adverb from mid-15c.