Quiz: Remember the definition of mal de mer?
c.1300, fede "enmity, hatred, hostility," northern English and Scottish; perhaps from an unrecorded Old English word or else from Old French fede, from Old High German fehida "contention, quarrel, feud," from Proto-Germanic *faihitha noun of state from adj. *faiho- (cf. Old English fæhð "enmity," fah "hostile;" German Fehde "feud;" Old Frisian feithe "enmity;" see foe). Sense of "vendetta" is early 15c. Alteration of spelling in 16c. is unexplained.
1670s, from feud (n.). Related: Feuded; feuding.