Online Etymology Dictionary

flush (v.1): mid-13c., flusshen "move rapidly or violently; rush, dart, spring" (intransitive); late 15c., flush up, transitive, "cause to fly; start or flush (birds)," perhaps imitative of the sound of beating wings.

The sense of "spurt, rush out suddenly, flow with force" (1540s, usually of water) probably is the same word, with the connecting notion being "sudden movement," but its senses seem more to fit the older ones of flash (v.), now all transferred to this word except in flash flood, via its variant flushe. OED considers this probably not connected to Old French flux. Transitive sense "cause to flow" is from 1590s.

Meaning "cleanse (a drain, etc.) with a rush of water" is from 1789. Of the face, "become suffused with warm color," from 1680s (flushed). Sense of "inflame with pride or passion" as a result of success, victory, etc., is from 1630s; perhaps influenced in sense by flesh (v). Related: Flushed; flushing.
flush (adj.): 1550s, "perfect, faultless;" c. 1600, "abundantly full," also "full of life or spirit," also "plentifully supplied" (with money, etc.), perhaps from flush (v.1) through the notion of a river running full, hence level with its banks. Meaning "even, level" is from 1620s, originally of ship's decks. In general use by 1791; in typography, 1900; in pugilism, 1812.
flush (n.): The section of entries for the various flushes in Century Dictionary opens with a caveat:
The several words spelled flush, being mostly dialectal, colloquial, or technical, and scantily recorded in early literature, have become partly confused with one another, and cannot now be entirely disentangled. Words originally different have acquired some meanings very nearly identical, while on the other hand there are some meanings not obviously related which are, nevertheless, to be referred to one original.
Weekley calls it "A very puzzling word." Sense of "a rush of water" in a stream (1520s), is probably from flush (v.1). From this likely come the extended senses "rush of emotion or passion" (1610s); "a sudden shooting up" (1773); "act of cleansing (a drain) by flushing" (1883); "glow of light or color" (especially sudden redness in the face), 1620s. Independently from the verb, probably, is the noun sense of "a flight of birds suddenly started up" (1590s).

The meaning "hand of cards all of one suit" (1520s) is of uncertain origin, perhaps formed on the model of Middle French flus (15c.), from Old French flux, flus "a flowing, rolling" (see flux), which, in common with its Italian cognate flusso, is said to have once had a sense of "a run" of cards. The form in English probably was influenced by flush (v.1).
flush (adv.): "directly, straight," 1700, from flush (adj.).
flush (v.2): "make even or level," 1842, from flush (adj.).
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), and thus perhaps distantly related to flush (v.1).
fluster (v.): early 15c. (implied in flostrynge), "bluster, agitate," probably from a Scandinavian source (compare Icelandic flaustr "bustle," flaustra "to bustle"), from Proto-Germanic *flaustra-. Originally "to excite," especially with drink; sense of "to flurry, confuse" is from 1724. Related: Flustered; flustering; flustery. As a noun, 1710, from the verb.
flustrated (adj.): 1712, jocular formation from fluster (v.) + frustrated. Related: Flustration.
flute (n.): early 14c., from Old French flaut, flaute (musical) "flute" (12c.), from Old Provençal flaut, which is of uncertain origin; perhaps imitative or from Latin flare "to blow" (see blow (v.1)); perhaps influenced by Provençal laut "lute." The other Germanic words (such as German flöte) are likewise borrowings from French.

Ancient flutes were direct, blown straight through a mouthpiece but held away from the player's mouth; 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.): "grooved, furrowed, ornamented," 1610s, past participle adjective from flute (v.).
flutist (n.): c. 1600, probably from French flûtiste (see flute (n.) + -ist); 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 (of birds), to fly before, flicker, float to and fro, be tossed by waves," frequentative of flotian "to float" (see float (v.)). Meaning "throw (someone) into confusion" is from 1660s. Related: Fluttered; fluttering. As a noun, "quick, irregular motion," from 1640s; meaning "state of excitement" is 1740s. Flutterpate "flighty person" is from 1894.
fluvial (adj.): "pertaining to a river," late 14c., from Latin fluvialis "of a river," from fluvius "a river, stream, running water," related to fluere "to flow" (see flow (v.)).
flux (n.): late 14c., "abnormally copious flow," from Old French flus "a flowing, a rolling; a bleeding" (Modern French flux), or directly from Latin fluxus (adj.) "flowing, loose, slack," past participle of fluere "to flow" (see fluent). Originally "excessive flow" (of blood or excrement), it also was 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 "a fly, winged insect," from Proto-Germanic *fleugon "flying insect" (source also of Old Saxon fleiga, Old Norse fluga, Middle Dutch vlieghe, Dutch vlieg, Old High German flioga, German Fliege "fly"); literally "the flying (insect)" (compare Old English fleogende "flying"), from same source as fly (v.1).

Originally any winged insect (moths, gnats, beetles, locusts, hence butterfly, etc.) and long used by farmers and gardeners for any insect parasite. Flies figuratively for "large numbers" of anything is from 1590s. Plural flien (as in oxen, etc.) gradually normalized 13c.-15c. to -s. Fly in the ointment is from Eccles. x:1. Fly on the wall "unseen observer" first recorded 1881. No flies on _____ "no lack of activity or alertness on the part of," is attested by 1866. Meaning "fish-hook dressed to resemble an insect" is from 1580s; Fly-fishing is from 1650s. Fly-catcher "bird which eats insects on the wing" is from 1670s. The fly agaric mushroom (1788) so called because it was used as a poison for flies.

The sense of "a flight, flying" is from mid-15c. From the verb and the notion of "flapping as a wing does" comes the noun sense of "tent flap" (1810), which was extended to "strip of material sewn into a garment as a covering for buttons" or some other purpose (1844). Baseball fly ball attested by 1866. To do something on the fly is 1856, apparently from baseball.
When the catcher sees several fielders running to catch a ball, he should name the one he thinks surest to take it, when the others should not strive to catch the ball on the fly, but only, in case of its being missed, take it on the bound. ["The American Boys Book of Sports and Games," New York, 1864]
fly (v.1): "to soar through air; move through the air with wings," Old English fleogan "to fly, take flight, rise into the air" (class II strong verb; past tense fleag, past participle flogen), from Proto-Germanic *fleugan "to fly" (source also of Old Saxon fliogan, Old Frisian fliaga, Middle Dutch vlieghen, Dutch vliegen, Old High German fliogan, German fliegen, Old Norse flügja), from PIE *pleuk-, extended form of *pleu- "to flow, float" (see pluvial).

Meaning "go at full speed" is from c. 1300. In reference to flags, 1650s. Transitive sense "cause to move or float in air" (as a flag, kite, etc.) is from 1739; sense of "convey through the air" ("Fly Me to the Moon") is from 1864. Related: Flew; flied (baseball); flown; flying. Slang phrase fly off the handle "lose one's cool" dates from 1825.
fly (v.2): "run away," Old English fleon, flion "fly from, avoid, escape;" essentially a variant spelling of flee (q.v.). In Old English, this verb and fleogan "soar through the air with wings" (modern fly (v.1)) differed only in their present tense forms and often were confused, then as now. In some Middle English dialects they seem to have merged completely. Distinguished from one another since 14c. in the past tense: flew for fly (v.1), fled for fly (v.2).
fly (adj.): slang, "clever, alert, wide awake," by 1811, 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 by Grose to be an old term of reproach to a woman signifying that she was a witch; used from 1823 in reference to anyone who departs hastily from a recent activity, especially while owing money. The different senses involve the two verbs fly.
fly-over (n.): also flyover, 1901 of bridges, 1931, of aircraft flights, from fly (v.1) + over (adv.).
fly-swatter (n.): in reference to a bit of wire mesh on a handle, 1917, from fly (n.) + agent noun from swat (v.). Simple swatter was used in this sense by 1906. Other older names for similar implements were fly-duster (1860), fly-whisk (1836), fly-brush (1823), fly-fan (1821), fly-flap (mid-15c., glossing Latin muscarium).
flyer (n.): also flier, mid-15c., "that which flies, thing or creature that flies," agent noun of fly (v.1). Meaning "something that goes fast" is from 1795. Meaning "speculative investment, financial venture" 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" (1916) developed in World War I. Related: Fliers; flyers.
flying (adj.): early 15c., replacing forms from Old English fleogende "flying, winged;" present participle adjective from fly (v.1). The meaning "attached so as to have freedom of movement" (1670s) is the source of the nautical use (flying jib, etc.). Meaning "designed for rapid movement" (especially in military terms) is from 1660s; meaning "passing, hasty, temporary, rapidly constructed" is from 1763.

Flying fish is from 1510s; flying buttress is from 1660s. Flying Dutchman, ghost ship off the Cape of Good Hope, is 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. Flying machine is from 1736 as a theoretical device. Flying saucer first attested 1947, though the image of saucers for unidentified flying objects is from at least 1880s.
Flynn: surname, from Irish flann "red." Rhyming phrase in like Flynn is 1940s slang, said to have originated in the U.S. military, perhaps from alleged sexual exploits of Hollywood actor Errol Flynn (1909-1959).
flypaper (n.): also fly-paper, 1851 (the thing 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." Flitecræft was used in Old English for "dialectics."
flyway (n.): "bird migration path," 1891, from fly (v.1) + way (n.).
flywheel (n.): also fly-wheel, "heavy-rimmed revolving wheel to regulate motion," 1784, from fly (n.) "speed-regulating device" (1590s, from fly (v.1)) + wheel (n.).
fo'c'sle (n.): see forecastle.
foal (n.): Old English fola "foal, colt," from Proto-Germanic *fulon (source also of Old Saxon folo, Middle Dutch volen, Dutch veulen, Old Norse foli, Old Frisian fola, Old High German folo, German Fohlen, Gothic fula), from PIE *pulo- "young of an animal" (source also of Greek polos "foal," Latin pullus "a young animal," Albanian pele "mare"), suffixed form of root *pau- (1) "few, little" (see few).
foal (v.): "give birth (to a foal)," late 14c., from foal (n.). Related: Foaled; foaling.
foam (n.): Middle English fom, fome (c. 1300), from Old English fam "foam, saliva froth; sea," from West Germanic *faimo- (source also of Old High German veim, German Feim), from PIE root *(s)poi-mo- "foam, froth" (source also of Sanskrit phenah; Latin pumex "pumice," spuma "foam;" Old Church Slavonic pena "foam;" Lithuanian spaine "a streak of foam"). The plastic variety used in packaging, etc., so called from 1937.
foam (v.): Old English famgian "to emit foam, to boil," from the source of foam (n.). Sense of "become foamy, to froth" is from late 14c. Transitive sense is from 1725. Related: Foamed; foaming.
foamy (adj.): Old English faemig "covered with foam;" see foam (n.) + -y (2). Related: Foaminess.
fob (n.): 1650s, "men's small waist pocket for valuables," of uncertain origin, probably related to Low German fobke "pocket," High German fuppe "pocket," "a dialectal word used in Livonia" [Klein]. Meaning "chain or ornament attached to a watch carried in the fob" is by 1888, shortened from fob chain.
fob (v.): "to cheat," late 14c., from obsolete noun fobbe "cheat, trickster" (late 14c.), which perhaps is from Old French forbeter "to deceive, trick, dupe." Alternative etymology holds that the word is perhaps related to German foppen "to jeer at, make a fool of" (see fop); or from German fuppen, einfuppen "to pocket stealthily," which would connect it to fob (n.).

Meaning "to put or shift off (something) by pretense" is from 1650s; to fob (someone) off "put him off deceitfully" is from 1590s. Related: Fobbed; fobbing.
focaccia (n.): by 1994, from Italian focaccia, from Late Latin focacia, fem. of focacius, used of breads baked under the ashes, from Latin focus "hearth, fireplace" (see focus (n.)). Cognate with Spanish hogaza, Old French foace "griddle cake" (Modern French fouasse "a cake, bun"), Provençal fogassa.
focal (adj.): "of or pertaining to a focus," 1690s, from Modern Latin focalis; see focus (n.) + -al (1).
foci (n.): classically correct plural of focus (n.).
focus (n.): 1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. Used in post-classical times for "fire" itself; taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to "center of activity or energy" is first recorded 1796.
focus (v.): 1775 in optics, "bring into focus" (transitive); 1807 in the figurative sense, from focus (n.). Intransitive use by 1864, originally in photography. Related: Focused; focusing; less commonly focussed; focussing.
fodder (n.): Old English fodder "food," especially "hay, straw, or other bulk food for cattle," from Proto-Germanic *fodram (source also of Old Norse foðr, Middle Dutch voeder, Old High German fuotar, German Futter), from PIE *pa-trom, suffixed form of *pa- "to feed" (see food).
foe (n.): Old English gefea, gefa "foe, enemy, adversary in a blood feud" (the prefix denotes "mutuality"), from adjective fah "at feud, hostile," also "guilty, criminal," from Proto-Germanic *faihaz (source also of Old High German fehan "to hate," Gothic faih "deception"), probably from PIE root *peig- (2) "evil-minded, treacherous, hostile" (source also of Sanskrit pisunah "malicious," picacah "demon;" Greek pikros "bitter;" Latin piget "it irks, troubles, displeases," piger "reluctant, lazy;" Lithuanian piktas "wicked, angry," pekti "to blame"). Weaker sense of "adversary" is first recorded c. 1600.
foe-man (n.): also foeman, "active enemy," late Old English fah-man; see foe + man (n.).
foetal (adj.): see fetal; for spelling, see oe.
foetid (adj.): see fetid; for spelling, see oe.
foetus (n.): see fetus; for spelling, see oe.
fog (n.1): "thick, obscuring mist," 1540s, a back-formation from foggy (which appeared about the same time) or from a Scandinavian source akin to Danish fog "spray, shower, snowdrift," Old Norse fjuk "drifting snow storm." Compare also Old English fuht, Dutch vocht, German Feucht "damp, moist." Figurative phrase in a fog "at a loss what to do" first recorded c. 1600. Fog-lights is from 1962.
fog (n.2): "long grass, second growth of grass after mowing," late 14c., probably of Scandinavian origin; compare Norwegian fogg "long grass in a moist hollow," Icelandic fuki "rotten sea grass." A connection to fog (n.1) via a notion of long grass growing in moist dells of northern Europe is tempting but not proven. Watkins suggests derivation from PIE *pu- (2) "to rot, decay" (see foul (adj.)).