What is the X in X-mas?
late 15c., herbengar "one sent ahead to arrange lodgings" (for a monarch, an army, etc.), alteration of Middle English herberger "provider of shelter, innkeeper" (late 12c.), from Old French herbergeor, from herbergier "provide lodging," from herber "lodging, shelter," from Frankish *heriberga "lodging, inn" (cf. Old Saxon, Old High German heriberga "army shelter," from heri "army" + berga "shelter"); see harbor. Sense of "forerunner" is mid-16c. Intrusive -n- is 15c. (see messenger). As a verb, from 1640s (harbinge "to lodge" is late 15c.).