mid-14c., reuerye, "wild conduct, frolic," from Old French reverie, resverie "revelry, rejoicing, wantonness, raving, delirium" (Modern French rêverie), from resver "to dream, wander, rave" (12c., Modern French rêver), a word of uncertain origin (also the source of rave).
The meaning "a daydream" also "fit of abstract musing, state of mental abstraction" is attested from 1650s, a reborrowing from French, which might explain why this old word in English has not been fully nativized as revery. "The most obvious external feature marking this state is the apparent unconsciousness or imperfect perception of external objects" [Century Dictionary]. As a type of instrumental musical composition, it is attested from 1880. Related: Reverist.