mid-14c., "warmth or glow of feeling," from Old French fervor "heat; enthusiasm, ardor, passion" (12c., Modern French ferveur), from Latin fervor "a boiling, violent heat; passion, ardor, fury," from fervere "to boil; be hot" (from PIE root *bhreu- "to boil, bubble, effervesce, burn").
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."
c. 1200 in reference to sexual possessiveness and suspicion, from Old French jalousie "enthusiasm, love, longing; jealousy" (12c.), from jalos "keen, zealous; avaricious; jealous" (see jealous). Also sometimes in Middle English in a sense "solicitude, carefulness, regard," the connecting notion being "watchfulness." Meaning "zeal, fervor, devotion" is from late 14c.
1906, named for U.S. president Theodore "Teddy" Roosevelt (1858-1919), a noted big-game hunter, whose conservationist fervor inspired a comic illustrated poem in the New York Times of Jan. 7, 1906, about two bears named Teddy, whose names were transferred to two bears presented to the Bronx Zoo that year. The name was picked up by toy dealers in 1907 for a line of "Roosevelt bears" imported from Germany. Meaning "big, lovable person" first attested 1957, from the song popularized by Elvis Presley.
c. 1600, from French enthousiasme (16c.) and directly from Late Latin enthusiasmus, from Greek enthousiasmos "divine inspiration, enthusiasm (produced by certain kinds of music, etc.)," from enthousiazein "be inspired or possessed by a god, be rapt, be in ecstasy," from entheos "divinely inspired, possessed by a god," from en "in" (see en- (2)) + theos "god" (from PIE root *dhes-, forming words for religious concepts). It acquired a derogatory sense of "excessive religious emotion through the conceit of special revelation from God" (1650s) under the Puritans; generalized meaning "fervor, zeal" (the main modern sense) is first recorded 1716.
Old English hætu, hæto "heat, warmth, quality of being hot; fervor, ardor," from Proto-Germanic *haita- "heat" (source also of Old Saxon hittia, Old Norse hiti, Old Frisian hete, German hitze "heat," Gothic heito "fever"), from the same source as Old English hat "hot" and hæða "hot weather" (see hot).
Meaning "a single course in a race," especially a horse race, is from 1660s, perhaps from earlier figurative sense of "violent action; a single intense effort" (late 14c.), or the meaning "run given to a horse to prepare for a race" (1570s). The latter word over time was extended to "division of a race or contest when there are too many contestants to run at once," the winners of each heat then competing in a final race.
Meaning "sexual excitement in animals" is from 1768, especially of females, corresponding to rut in males. Meaning "trouble with the police" attested by 1920. Heat wave "period of excessive hot weather" first attested 1890; earlier in reference to solar cycles. Heat-stroke is from 1858. Heat-seeking (adj.) of missiles, etc., is by 1955. Red heat, white heat are in reference to the color of heated metals, especially iron.
"a member of the Christian denomination known as the Religious Society of Friends," 1651, said to have been applied to them in 1650 by Justice Bennett at Derby, from George Fox's admonition to his followers to "tremble at the Word of the Lord;" but the word was used earlier of foreign sects given to fits of shaking during religious fervor, and that is likely the source here. Either way, it never was an official name of the Religious Society of Friends.
The word in a literal sense of "one who or that which trembles" is attested from early 15c., an agent noun from quake (v.). The notion of "trembling" in religious awe is in Old English; quaking (n.) meaning "fear and reverence" especially in religion is attested from mid-14c.
There is not a word in the Scripture, to put David's condition into rime and meeter: sometimes he quaked and trembled, and lay roaring all the day long, that he watered his bed with his tears: and how can you sing these conditions (but dishonour the Lord) and say all your bones quake, your flesh trembled, and that you water your bed with your tears? when you live in pride and haughtiness, and pleasure, and wantonness .... ["A Brief Discovery of a threefold estate of Antichrist Now Extant in the world, etc.," an early Quaker work, London, 1653]
Figuratively, as an adjective, in reference to plain or drab colors (such as were worn by members of the sect) is by 1775. A Quaker gun (1809, American English), originally a log painted black and propped up to resemble the barrel of a cannon to deceive the enemy from a distance, is so called for the sect's noted pacifism. Philadelphia, Pennsylvania, has been known as the Quaker City at least since 1824. Related: Quakerish; Quakeress ("a female Quaker"); Quakerism; Quakerdom; Quakerly.