Etymology
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fervid (adj.)
1590s, "burning, glowing, hot," from Latin fervidus "glowing, burning; vehement, fervid," from fervere "to boil, glow" (from PIE root *bhreu- "to boil, bubble, effervesce, burn"). Figurative sense of "impassioned" is from 1650s. Related: Fervidly; fervidness.
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perfervid (adj.)

"very hot, very ardent," 1830, as if from Latin *perfervidus, from per "completely" (see per) + fervidus "glowing, burning; vehement" (see fervid). Related: Perfervidly.

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*bhreu- 
also *bhreuə-, *bhreəu-, Proto-Indo-European root meaning "to boil, bubble, effervesce, burn," with derivatives referring to cooking and brewing.

It forms all or part of: barm; barmy; bourn (n.1) "small stream;" braise; bratwurst; brawn; brawny; braze (v.1) "to expose to the action of fire;" brazier; Brazil; bread; breed; brew; broth; broil (v.2) "to quarrel, brawl;" brood; effervesce; effervescence; effervescent; embroil; ferment; fervent; fervid; fervor; imbroglio.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhurnih "violent, passionate;" Greek phrear "well, spring, cistern;" Latin fervere "to boil, foam," Thracian Greek brytos "fermented liquor made from barley;" Russian bruja "current;" Old Irish bruth "heat;" Old English breowan "to brew," beorma "yeast;" Old High German brato "roast meat."
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