What is the X in X-mas?
mid-14c., hengestman, later henshman (mid-15c.) "high-ranking servant (usually of gentle birth), attendant upon a king, nobleman, etc.," originally "groom," probably from man (n.) + Old English hengest "horse, stallion, gelding," from Proto-Germanic *hangistas (cf. Old Frisian hengst, Dutch hengest, German Hengst "stallion"), perhaps literally "best at springing," from PIE *kenku- (cf. Greek kekiein "to gush forth;" Lithuanian sokti "to jump, dance;" Breton kazek "a mare," literally "that which belongs to a stallion").
Perhaps modeled on Old Norse compound hesta-maðr "horse-boy, groom." The word became obsolete in England but was retained in Scottish as "personal attendant of a Highland chief," in which sense Scott revived it in literary English from 1810. Sense of "obedient or unscrupulous follower" is first recorded 1839, probably based on a misunderstanding of the word as used by Scott.