Online Etymology Dictionary

flextime (n.): also short for flexitime, 1972, translating German Gleitzeit "sliding time." See flex + time (n.).
flexuous (adj.): "full of bends or curves, winding, sinuous," c. 1600, from Latin flexuosus, from flexus (n.) "a bending," from flectere "to bend" (see flexible). From 1620s as "undulating."
flexure (n.): 1590s, "action of flexing or bending," from Latin flextura, from flectere "to bend" (see flexible). From 1620s as "flexed or bent condition; direction in which something is bent." Picked up in mathematics (1670s), geology (1833).
flibbertigibbet (n.): 1540s, "chattering gossip, flighty woman," probably a nonsense word meant to sound like fast talking; as the name of a devil or fiend it dates from c. 1600 (together with Frateretto, Hoberdidance, Tocobatto). OED lists 15 spellings and thinks flibbergib is the original.
flick (n.): mid-15c., "light blow or stroke," probably imitative of a light blow with a whip. Earliest recorded use is in phrase not worth a flykke "useless." Meaning "quick turn of the wrist" is from 1897 in sports. As slang for "film," it is first attested 1926, a back-formation from flicker (v.), from their flickering appearance.
flick (v.): 1816, "to throw off with a jerk," from flick (n.). Meaning "strike lightly with a quick jerk" is from 1838. Related: Flicked; flicking.
flicker (v.): Old English flicorian "to flutter, flap quickly and lightly, move the wings," originally of birds. Onomatopoeic and suggestive of quick motion. Sense of "shine with a wavering light" is c. 1600, but not common till 19c. Related: Flickered; flickering.
flicker (n.2): type of North American woodpecker, 1808, American English, said to be echoic of bird's note, or from black spots on plumage of the underparts that seem to flicker as it flits from tree to tree.
flicker (n.1): 1849, "wavering, unsteady light or flame;" 1857 as "a flickering," from flicker (v.).
flier (n.): see flyer.
flight (n.1): "act of flying," Old English flyht "a flying, act or power of flying," from Proto-Germanic *flukhtiz (source also of Dutch vlucht "flight of birds," Old Norse flugr, Old High German flug, German Flug "flight"), from Proto-Germanic *flug-ti-, from PIE *pluk-, from root *pleu- "to flow" (see fly (v.1)).

Spelling altered late 14c. from Middle English fliht (see fight (v.)). Sense of "swift motion" is from mid-13c.. Meaning "an instance of flight" is 1785, originally of ballooning. Sense of "a number of things passing through the air together" is from mid-13c. Meaning "series of stairs between landings" is from 1703. Figuratively, "an excursion" of fancy, imagination, etc., from 1660s. Flight-path is from 1908; flight-test (v.) from 1919; flight-simulator from 1947 (originally in rocketry); flight-attendant from 1946.
flight (n.2): "act of fleeing," c. 1200, flihht, not found in Old English, but presumed to have existed and cognate with Old Saxon fluht, Old Frisian flecht "act of fleeing," Dutch vlucht, Old High German fluht, German Flucht, Old Norse flotti, Gothic þlauhs, from Proto-Germanic *flug-ti- (see flight (n.1)). To put (someone or something) to flight "rout, defeat" is from late 14c., the earlier verb form do o' flight (early 13c.).
flightless (adj.): "incapable of flying," 1846, from flight (n.1) + -less. Related: Flightlessly; flightlessness.
flighty (adj.): 1550s, "swift," from flight (n.1) + -y (2). Sense of "fickle or frivolous" is from 1768, perhaps from notion of "given to 'flights' of imagination." Related: Flightiness.
flim-flam (n.): also flimflam, 1530s, a contemptuous echoic construction, perhaps connected to some unrecorded dialectal word from Scandinavian (compare Old Norse flim "a lampoon"). From 1650s as a verb. Related: Flim-flammer.
flimsy (adj.): 1702, of unknown origin, perhaps a metathesis of film (n.) "gauzy covering" + -y (2). Figuratively (of arguments, etc.) from 1750s. Related: Flimsily; flimsiness.
flinch (v.): 1570s, apparently a nasalized form of obsolete Middle English flecche "to bend, flinch," which probably is from Old French flechir "to bend" (Modern French fléchir), also flechier "to bend, turn aside, flinch," which probably are from Frankish *hlankjan or some other Germanic source, from Proto-Germanic *hlinc- (source also of Middle High German linken, German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn" (see link (n.)). There were nasalized form of the word in Old French as well (flenchir "to bend; give ground, retreat"). Related: Flinched; flinching. As a noun, "the action of flinching," from 1817.
flinders (n.): "pieces, fragments, splinters," mid-15c., Scottish flendris, probably related to Norwegian flindra "chip, splinter," or Dutch flenter "fragment;" ultimately from the same PIE root that produced flint.
fling (v.): c. 1300, "to dash, run, rush," probably from or related to Old Norse flengja "to flog," which is of uncertain origin, perhaps from Proto-Germanic *flang- (source also of Old Swedish flenga "strike," Danish flænge "slash, gash"), from a nasalized variant of PIE *plak- (2) "to strike" (see plague (n.)). Meaning "to throw, cast, hurl" is from mid-14c. An obsolete word for "streetwalker, harlot" was fling-stink (1670s). Related: Flung; flinging, but in Middle English with past tense flang, past participle flungen.
fling (n.): early 14c., "attempt, attack," (in phrase make a fling), from fling (v.). Hence have a fling at, etc. "make a try." From 1560s as "a wild dash, an excited kicking up." Sense of "period of indulgence on the eve of responsibilities" first attested 1827. Meaning "vigorous dance" (associated with the Scottish Highlands) is from 1804.
flint (n.): Old English flint "flint; a type of rock noted for hardness and for giving off sparks when struck," common Germanic (cognates Middle Dutch vlint, Old High German flins, Danish flint), from PIE *splind- "to split, cleave," from root *(s)plei- "to splice, split" (source also of Greek plinthos "brick, tile," Old Irish slind "brick"), perhaps a variant of *spel- (1) "to split, break off." Transferred senses (hardness, etc.) were in Old English.
flint-lock (n.): also flintlock, 1680s as a type of gunlock in which fire is produced by a flint striking the hammer, from flint + lock (n.1).
flintstone (n.): "hard silicious stone, flint," early 14c., from flint + stone (n.).
flinty (adj.): 1530s, "hard-hearted;" 1540s, "hard, impenetrable as flint," from flint + -y (2). Literal sense of "resembling flint" is from 1640s. Related: Flintily; flintiness.
flip (v.): 1590s "to fillip, to toss with the thumb," imitative, or perhaps a thinned form of flap, or else a contraction of fillip (q.v.), which also is held to be imitative. Meaning "toss as though with the thumb" is from 1610s. Meaning "to flip a coin" (to decide something) is by 1879. Sense of "get excited" is first recorded 1950; flip (one's) lid "lose one's head, go wild" is from 1949, American English; variant flip (one's) wig attested by 1952, but the image turns up earlier in popular record reviews ["Talking Boogie. Not quite as wig-flipping as reverse side--but a wig-flipper" Billboard, Sept. 17, 1949]. Related: Flipped. Flipping (adj.) as euphemism for fucking is British slang first recorded 1911 in D.H. Lawrence. Flip side (of a gramophone record) is by 1949.
flip (n.2): sailors' hot drink usually containing beer, brandy and sugar, 1690s, from flip (v.); so called from notion of it being "whipped up" or beaten.
flip (adj.): "talkative and disrespectfully smart," see flippant.
flip (n.1): 1690s, "a flick, a snap;" see flip (v.). In reference to an overturning of the body, probably short for flip-flap (see flip-flop) "somersault in which the performer throws himself over on hands and feet alternately," 1670s, originally a move in (male) dancing.
flip-flop (n.): also flip flop, "plastic thong beach sandal," by 1970, imitative of the sound of walking in them. Flip-flap had been used in various senses, mostly echoic or imitative of a kind of loose flapping movement, since 1520s:
Flip-flaps, a peculiar rollicking dance indulged in by costermongers, better described as the double shuffle; originally a kind of somersault. [Hotten's Slang Dictionary, 1864]
Flip-flop in the general sense of "complete reversal of direction" dates from 1900; it began to be used in electronics in the 1930s in reference to switching circuits that alternate between two states. As a verb by 1897. Flop (n.) in the sense "a turn-round, especially in politics" is from 1880.
flip-top (adj.): 1955, of product packaging, from flip (v.) + top (n.1).
flippancy (n.): 1746, from flippant + -cy.
flippant (adj.): c. 1600, "talkative, nimble in talk;" 1670s, "displaying unbecoming levity," apparently an extended form of flip (v.). The ending is perhaps modeled on other adjectives in -ant or a relic of the Middle English present participle ending -inde. Shortened form flip is attested from 1847. Related: Flippantly.
flipper (n.): limb used to swim with, 1822, agent noun from flip (v.). Sense of "rubber fin for underwater swimming" is from 1945. Slang meaning "the hand" dates from 1836. Related: Flippers.
flirt (v.): 1550s, "to turn up one's nose, sneer at;" later "to rap or flick, as with the fingers" (1560s); "throw with a sudden movement," also "move in short, quick flights" (1580s). Perhaps imitative (compare flip (v.), also East Frisian flirt "a flick or light blow," flirtje "a giddy girl," which also might have fed into the English word), but perhaps rather from or influenced by flit (v.). Related: Flirted; flirting.

The main modern verbal sense of "play at courtship" (1777) probably developed from the noun (see flirt (n.)) but also could have grown naturally from the 16c. meaning "to flit inconstantly from object to object." To flirt a fan (1660s) was to snap it open or closed with a brisk jerk and was long considered part of the coquette's arsenal, which might have contributed to the sense shift. Or the word could have been influenced from French, where Old French fleureter meant "talk sweet nonsense," also "to touch a thing in passing," diminutive of fleur "flower" (n.) and metaphoric of bees skimming from flower to flower. French flirter "to flirt" is a 19c. borrowing from English.
flirt (n.): 1540s, "joke, jest, stroke of wit, contemptuous remark," from flirt (v.). By 1560s as "a pert young hussey" [Johnson], and Shakespeare has flirt-gill (i.e. Jill) "a woman of light or loose behavior" (Fletcher formalizes it as flirt-gillian), while flirtgig was a 17c. Yorkshire dialect word for "a giddy, flighty girl." One of the many fl- words suggesting loose, flapping motion and connecting the notions of flightiness and licentiousness. Compare English dialect and Scottish flisk "to fly about nimbly, skip, caper" (1590s); source of Scott's fliskmahoy "girl giddy and full of herself." The meaning "person who plays at courtship" is from 1732 (as the name of female characters in plays at least since 1689 (Aphra Behn's "The Widow Ranter")). Also in early use sometimes "person one flirts with," though by 1862 this was being called a flirtee.
flirtation (n.): "amorous trifling; giddy behavior," 1718, noun of action from flirt (v.) as though Latin.
flirtatious (adj.): 1834, from flirtation + -ous. Related: Flirtatiously; flirtatiousness.
flirty (adj.): 1840, from flirt (v.) + -y (2). Related: Flirtiness.
flit (v.): c. 1200, flitten, flytten, flutten "convey, move (a thing) from one place to another, take, carry away," also intransitive, "go away, move, migrate," from Old Norse flytja "to remove, bring," from Proto-Germanic *flutjan- "to float," from extended form of PIE *pleu- "to flow" (see pluvial). Intransitive sense "move lightly and swiftly" is from early 15c.; from c. 1500 as "remove from one habitation to another" (originally Northern English and Scottish)
Theire desire ... is to goe to theire newe masters eyther on a Tewsday, or on a Thursday; for ... they say Munday flitte, Neaver sitte. [Henry Best, farming & account book, 1641]
Related: Flitted; flitting. As a noun, "a flitting, a removal," from 1835.
flitch (n.): "side of bacon," Middle English flicche (c. 1200), "side of a slaughtered animal," especially the salted and cured side of a hog, from Old English flicce "flitch of bacon, ham," from Proto-Germanic *flekkja (source also of Old Norse flikki, Middle Low German vlicke "piece of flesh"). Not immediately from flesh (n.), but perhaps from the same PIE root, *pleik- "to tear" (see flay). The Flitch of Dunmow was presented every year at Little Dunmow, in Essex, to any married couple who could prove they had lived together without quarreling for a year and a day, a custom mentioned in early references as dating to mid-13c., revived 19c.
flite (v.): "to scold," c. 1500, earlier "to content with words, chide, wrangle," from Old English flitan, cognate with Old High German flizzan "to strive." Related: Flited; fliting.
flitter (v.): "fly with back-and-forth motion," 1540s, from flit with frequentative suffix. Flitter-mouse (1540s) is occasionally used in English, in imitation of German fledermaus "bat," from Old High German fledaron "to bat, to flutter." Related: Flittered; flittering. As a noun, from 1892.
flitty (adj.): 1640s, from flit (n.) + -y (2). Related: Flittiness.
flivver (n.): "cheap car," especially "Model-T Ford," 1910, of unknown origin.
float (v.): late Old English flotian "to rest on the surface of water" (intransitive; class II strong verb; past tense fleat, past participle floten), from Proto-Germanic *flotan "to float" (source also of Old Norse flota, Middle Dutch vloten, Old High German flozzan, German flössen), from *flot-, from PIE *pleud-, extended form of root *pleu- "to flow" (see pluvial).

Meaning "drift about, hover passively" is from c. 1300. Transitive sense of "to lift up, cause to float" (of water, etc.) is from c. 1600; that of "set (something) afloat" is from 1778 (originally of financial operations). Of motion through air, from 1630s. Meaning "hover dimly before the eyes" is from 1775. Related: Floated; floating. A floating rib (by 1802) is so called because the anterior ends are not connected to the rest.
float (n.): apparently an early Middle English merger of three related Old English nouns, flota "boat, fleet," flote "troop, flock," flot "body of water, sea;" all from the source of float (v.). The early senses were the now-mostly-obsolete ones of the Old English words: "state of floating" (early 12c.), "swimming" (mid-13c.); "a fleet of ships; a company or troop" (c. 1300); "a stream, river" (early 14c.). From c. 1300 as an attachment for buoyancy on a fishing line or net; early 14c. as "raft." Meaning "platform on wheels used for displays in parades, etc." is from 1888, probably from earlier sense of "flat-bottomed boat" (1550s). As a type of fountain drink, by 1915.
Float.--An ade upon the top of which is floated a layer of grape juice, ginger ale, or in some cases a disher of fruit sherbet or ice cream. In the latter case it would be known as a "sherbet float" or an "ice-cream float." ["The Dispenser's Formulary: Or, Soda Water Guide," New York, 1915]

Few soda water dispensers know what is meant by a "Float Ice Cream Soda." This is not strange since the term is a coined one. By a "float ice cream soda" is meant a soda with the ice cream floating on top, thus making a most inviting appearance and impressing the customer that you are liberal with your ice cream, when you are not really giving any more than the fellow that mixes his ice cream "out of sight." ["The Spatula," Boston, July, 1908]
floatation (n.): 1806, the older, more etymological, but less popular spelling of flotation.
floater (n.): "one who or that which floats," 1717, agent noun from float (v.). From 1847 in political slang for an independent voter (but with suggestion of purchasability); 1859 as "one who frequently changes place of residence or employment." Meaning "dead body found in water" is 1890, U.S. slang.
floc (n.): 1921, "mass of fine particles," diminutive of flocculus (see flocculate).
floccinaucinihilipilification (n.): "action or habit of estimating as worthless," in popular smarty-pants use from c. 1963; attested 1741 (in a letter by William Shenstone, published 1769), a combination of four Latin words (flocci, nauci, nihili, pilifi) all signifying "at a small price" or "for nothing," which appeared together in a rule of the well-known Eton Latin Grammar.
[F]or whatever the world might esteem in poor Somervile, I really find, upon critical enquiry, that I loved him for nothing so much as his flocci-nauci-nihili-pili-fication of money. [Shenstone, letter, 1741]
The kind of jocular formation that was possible among educated men in Britain in those days. Just so, as in praesenti, the opening words of mnemonic lines on conjugation in Lilley's 16c. Latin grammar, could stand alone as late as 19c. and be understood to mean "rudiments of Latin."