What is the X in X-mas?
mid-14c., reuerye, "wild conduct, frolic," from Old French reverie, resverie "revelry, raving, delirium" (Modern French rêverie), from resver "to dream, wander, rave" (12c., Modern French rêver), of uncertain origin (also the root of rave). Meaning "daydream" is first attested 1650s, a reborrowing from French. As a type of musical composition, it is attested from 1880. Related: Reverist.