What is the X in X-mas?
late 14c., "uncastrated male horse," from Old French estalon "male horse," from Frankish *stal, cognate with Old High German stal "stable" (see stall (n.1)). The notion is probably of a horse kept in a stable to service mares. Transferred sense of "lascivious man" is attested from 1550s.
[fr Old English steall, ''standing, state, place, animal stall,'' whence the notion of stubbornly holding one's place]