What is the X in X-mas?
mid-14c., from a Scandinavian source (cf. Norwegian skule "look furtively, squint, look embarrassed," Danish skule "to scowl, cast down the eyes"). Probably related to Old English sceolh "wry, oblique," Old High German scelah "curved," German scheel "squint-eyed;" from PIE root *sqel- "crooked, curved, bent." Related: Scowled; scowling.
c.1500, from scowl (v.).