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mid-14c., from Old French fervent, from Latin ferventem (nominative fervens) "boiling, hot, glowing," figuratively "violent, impetuous, furious," present participle of fervere "to boil, glow," from PIE root *bhreue- (see brew). The figurative sense of "impassioned" is first attested c.1400. Related: Fervency; fervently.