farewell (interj.)

expression at parting, late 14c., from Middle English faren wel, verbal phrase attested by c. 1200 (see fare (v.) + well (adv.)); usually said to the departing person, who replied with good-bye. As a noun, "a good-bye, a leave-taking," by early 15c. Expression to a fare-thee-well "to the last degree" is by 1884, American English.