Online Etymology Dictionary

fluffy (adj.): 1825, from fluff + -y (2). Related: Fluffiness.
flugelhorn (n.): 1854, from German flügelhorn, from flügel "wing" (related to fliegen "to fly;" see fly (v.1)) + horn "horn" (see horn (n.)).
fluid (adj.): early 15c., from Middle French fluide (14c.) and directly from Latin fluidus "fluid, flowing, moist," from fluere "to flow" (see fluent). Figurative use from 1640s. Related: Fluidly.
fluid (n.): 1660s, from fluid (adj.).
fluidity (n.): c.1600, from French fluidité, from fluide (see fluid (adj.)), or else formed in English from fluid.
fluke (n.1): "flat end of an arm of an anchor," 1560s, perhaps from fluke (n.3) on resemblance of shape, or from Low German flügel "wing." Meaning "whale's tail" (in plural, flukes) is 1725.
fluke (n.2): "lucky stroke, chance hit," 1857, originally a lucky shot at billiards, of uncertain origin.
fluke (n.3): "flatfish," Old English floc "flatfish," related to Old Norse floke "flatfish," flak "disk, floe" (see flake (n.)). The parasite worm (1660s) so called from resemblance of shape.
fluky (adj.): 1867, from fluke (n.2) + -y (2).
flume (n.): late 12c., "stream," from Old French flum "running water, stream, river," from Latin flumen "flood, stream, running water," from fluere "to flow" (see fluent). In U.S., used especially of artificial streams channeled for some industrial purpose.
flummery (n.): 1620s, a type of coagulated food, from Welsh llymru "sour oatmeal jelly boiled with the husks," of uncertain origin. Figurative use, of flattery, empty talk, is from 1740s.
flummox (v.): 1837, cant word, origin uncertain, probably from some forgotten British dialect. Candidates cluster in Herefordshire, Gloucestershire, southern Cheshire and also in Sheffield. "The formation seems to be onomatopœic, expressive of the notion of throwing down roughly and untidily." Never let it be said that the OED editors lacked imagination. Related: Flummoxed.
flung: past participle of fling (v.).
flunk (v.): 1823, American English college slang, original meaning "to back out, give up, fail," traditionally said to be an alteration of British university slang funk "to be frightened, shrink from" (see funk (n.1)). Related: Flunked; flunking.
flunky (n.): also flunkey, 1782, Scottish dialect, "footman, liveried servant," of uncertain origin, perhaps a diminutive variant of flanker. Sense of "flatterer, toady" first recorded 1855.
fluor (n.): 16c., and old chemistry term for "minerals which were readily fusible and useful as fluxes in smelting" [Flood], Latin fluor, originally meaning "a flowing, flow" (see fluent). Since 1771 applied to minerals containing fluorine, especially calcium fluoride (fluorspar or fluorite).
fluorescence (n.): 1852, "glowing in ultraviolet light," coined by English mathematician and physicist Sir George G. Stokes (1819-1903) from fluorspar (see fluorine), because in it he first noticed the phenomenon, + -escence, on analogy of phosphorescence.
fluorescent (adj.): 1853, from fluor- + -escent (see fluorescence). The fluorescent electric lamp patent was applied for in 1896 by U.S. inventor Thomas A. Edison, but such lights were rare before 1938.
fluoridate (v.): 1949, back-formation from fluoridation. Related: Fluoridated; fluoridating.
fluoridation (n.): 1904, from fluoride + -ation. In reference to adding traces of fluoride to drinking water as a public health policy, from 1949.
fluoride (n.): 1826, "binary compound of fluorine with another element," from fluorine + -ide.
fluorine (n.): non-metallic element, 1813, coined by English chemist Sir Humphry Davy (1778-1829) from fluorspar ("calcium fluoride," modern fluorite), the late 18c. name of the mineral where it was first found; see fluor + chemical suffix -ine (2). Not isolated until 1886.
fluoroscopy (n.): 1896, from fluor- (see fluorescence) + -oscopy (see -scope).
flurry (n.): "snow squall" 1828, American English, with earlier senses of "commotion," etc., dating to 1680s; perhaps imitative, or else from 17c. flurr "to scatter, fly with a whirring noise," perhaps from Middle English flouren "to sprinkle, as with flour" (late 14c.).
flurry (v.): 1757 in the commotion sense, from flurry (n.); 1883 in the snow sense. Related: Flurried; flurries; flurrying.
flush (v.): "fly up suddenly," c.1300, perhaps imitative of the sound of beating wings, or related to flash via its variant flushe. Probably not connected to Old French flux, source of flush (n.).

Transitive meaning "to cause to fly, start" is first attested mid-15c. The sense of "spurt, rush out suddenly, flow with force" (1540s) is probably the same word, with the connecting notion being "sudden movement," but its senses seem more to fit the older ones of flash (now all transferred to this word except in flash flood). Meaning "cleanse a drain, etc., with a rush of water" is from 1789. The noun sense of "sudden redness in the face" (1620s) probably belongs here, too. The verb in this sense is from 1660s. "A very puzzling word" [Weekley]. Related: Flushed; flushing.
flush (adj.): 1550s, "perfect, faultless;" c.1600, "abundant; plentifully supplied (with money, etc.)," perhaps from flush (v.) through the notion of a river running full, hence level with its banks. Meaning "even, level" is from 1620s.
flush (n.): "hand of cards all of one suit," 1520s, perhaps from Middle French flus (15c.), from Old French flux "a flowing," with the sense of "a run" (of cards), from Latin fluxus "flux," from fluere "to flow" (see fluent). The form in English probably was influenced by flush (v.).
Flushing: New York village established 1645 by English Puritans (now a neighborhood in Queens), an English corruption of Dutch Vlissingen, name of Dutch town where the Puritans had taken refuge, literally "flowing" (so called for its location on an estuary of the West Scheldt), related to flush (v.).
fluster (v.): early 15c. (implied in flostyrynge), from a Scandinavian source (cf. Icelandic flaustr "bustle," flaustra "to bustle"). Originally "to excite," especially with drink; sense of "to flurry, confuse" is from 1724. Related: Flustered; flustering. As a noun, 1710, from the verb.
flustrated (adj.): 1712, jocular formation from fluster + frustrated.
flute (n.): early 14c., from Old French flaute (12c.), from Old Provençal flaut, of uncertain origin, perhaps imitative or from Latin flare "to blow;" perhaps influenced by Provençal laut "lute." The other Germanic words (cf. German flöte) are likewise borrowings from French.

Ancient flutes were blown through a mouthpiece, like a recorder; the modern transverse or German flute developed 18c. The older style then sometimes were called flûte-a-bec (French, literally "flute with a beak"). The modern design and key system of the concert flute were perfected 1834 by Theobald Boehm. The architectural sense of "furrow in a pillar" (1650s) is from fancied resemblance to the inside of a flute split down the middle. Meaning "tall, slender wine glass" is from 1640s.
flute (v.): late 14c., "to play upon the flute," from flute (n.). Meaning "to make (architectural) flutes" is from 1570s. Related: Fluted; fluting.
fluted (adj.): 1610s, past participle adjective from flute (v.).
flutist (n.): c.1600, probably from French flûtiste; replaced Middle English flouter (early 13c., from Old French flauteor) and is preferred in U.S. The British preference is flautist (q.v.), a Continental reborrowing that returns the original diphthong.
flutter (v.): Old English floterian "to flutter, fly, flicker, float to and fro, be tossed by waves," frequentative of flotian "to float" (see float (v.)). Related: Fluttered; fluttering. As a noun from 1640s; meaning "state of excitement" is 1740s.
fluvial (adj.): "pertaining to a river," late 14c., from Latin fluvialis "of a river," from fluvius "river," related to fluere "to flow" (see flow (v.)).
flux (n.): late 14c., from Old French flus "flowing, rolling, bleeding," or directly from Latin fluxus "flowing, loose, slack," past participle of fluere "to flow" (see fluent). Originally "excessive flow" (of blood or excrement); an early name for "dysentery;" sense of "continuous succession of changes" is first recorded 1620s. The verb is early 15c., from the noun.
fly (n.): Old English fleoge "fly, winged insect," from Proto-Germanic *fleugjon (cf. Old Saxon fleiga, Old Norse fluga, Middle Dutch vlieghe, Dutch vlieg, Old High German flioga, German Fliege "fly); literally "the flying (insect)" (cf. Old English fleogende "flying"), from same source as fly (v.1).

Originally any winged insect (hence butterfly, etc.); long used by farmers and gardeners for any insect parasite. The Old English plural in -n (cf. oxen) gradually normalized 13c.-15c. to -s. Fly on the wall "unseen observer" first recorded 1881. An Old English word for "curtain" was fleonet "fly-net." Fly-swatter first attested 1917. Fly-fishing is from 1650s.
fly (v.1): "to soar through air," Old English fleogan "to fly" (class II strong verb; past tense fleag, past participle flogen), from West Germanic *fleuganan (cf. Old Saxon and Old High German fliogan, Old Norse flügja, Old Frisian fliaga, Middle Dutch vlieghen, Dutch vliegen, German fliegen), from PIE *pleu- "flowing, floating" (see pluvial).

Notion of "flapping as a wing does" led to noun sense of "tent flap" (1810), which yielded (1844) "covering for buttons that close up a garment." The noun sense of "a flight, flying" is from mid-15c. Baseball fly ball attested by 1866. Slang phrase fly off the handle "lose one's cool" dates from 1825. To do something on the fly is 1856, apparently from baseball.
fly (v.2): "run away," Old English fleon (see flee). Fleogan and fleon were often confused in Old English, too. Modern English distinguishes in preterite: flew/fled.
fly (adj.): slang, "clever, alert, wide awake," late 18c., perhaps from fly (n.) on the notion of the insect being hard to catch. Other theories, however, trace it to fledge or flash. Slang use in 1990s might be a revival or a reinvention.
fly-by-night (n.): 1796, slang, said to be an old term of reproach to a woman signifying that she was a witch; extended 1823 to "anyone who departs hastily from a recent activity," especially while owing money. The two senses involve the two verbs fly.
fly-over (n.): also flyover, 1901 of bridges, 1931, of aircraft flights, from fly (v.1) + over (adv.).
flyer (n.): also flier, mid-15c., "something that flies," agent noun of fly (v.1). Meaning "something that goes fast" is from 1795; that of "aviator" is from 1934. Meaning "speculative investment" is from 1846 (on the notion of a "flying leap"). Meaning "small handbill or fly-sheet" is from 1889, U.S. slang (originally especially of police bulletins), on notion of "made to be scattered broadcast." Meaning "aviator" is from World War I. Related: Fliers; flyers.
flying (adj.): Old English fleogende "flying, winged," present participle of fly (v.1). Flying buttress is from 1660s; flying fish is from 1510s. Flying saucer first attested 1947, though the image of saucers for unidentified flying objects is from at least 1880s. Flying Dutchman, ghost ship off the Cape of Good Hope, attested since 1803 [John Leyden, "Scenes of Infancy," who describes it as "a common superstition of mariners"]. Flying colors (1706) probably is from the image of a naval vessel with the national flag bravely displayed.
Flynn: surname, from Irish flann "red."
flypaper (n.): 1851 (though the item itself is said to have become commonly available in London in 1848), from fly (n.1) + paper (n.).
flyte (v.): Old English flitan "to contend, struggle, quarrel;" related to German fleiß, Dutch vlijt "diligence, industry."
flyway (n.): 1891, originally of bird migration paths, from fly (v.1) + way (n.).