fervent: [14] Fervent comes from the present participle of Latin fervēre ‘boil’. This verb also produced English effervescent [17] and comfrey [15], a plant-name which means literally ‘boil together’, and moreover its derivative fermentum led to English ferment [14]. It goes back ultimately to the Indo-European base *bhreu- or *bhru-, from which English also gets brew, broth, and fry, and possibly bread and burn. => brew, broth, comfrey, effervescent, ferment, fry
fervent (adj.)
mid-14c., from Old French fervent "fervent, ardent" (12c.), from Latin ferventem (nominative fervens) "boiling, hot, glowing," figuratively "violent, impetuous, furious," present participle of fervere "to boil, glow," from PIE root *bhreuə- "to boil, bubble" (see brew (v.)). The figurative sense of "impassioned" is first attested c. 1400. Related: Fervency; fervently.