Quiz: Remember the definition of mal de mer?
mid-15c. (adjective and noun), from Middle French infidèle, from Latin infidelis "unfaithful, not to be trusted," later "unbelieving," from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (see fidelity). In 15c. "a non-Christian" (especially a Saracen); later "one who does not believe in religion" (1520s). Also used to translate Arabic qafir, which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s; see kaffir).