Online Etymology Dictionary

flea: O.E. fleah, from P.Gmc. *flaukh- (cf. O.N. flo, M.Du. vlo, Ger. floh), perhaps related to O.E. fleon "to flee," with a notion of "the jumping parasite."
flea market: 1917, especially in reference to the Fr. marché aux puces in Paris, so-called "because there are so many second-hand articles sold of all kinds that they are believed to gather fleas."
fleck (v.): early 15c., from O.N. flekka "to spot," from P.Gmc. *flekk- (cf. M.Du. vlecke, O.H.G. flec, Ger. Fleck). Related: Flecked; flecking. The noun is first recorded 1590s, probably from the verb.
fledge: O.E. flycge (Kentish fligge), from W.Gmc. *fluggja- (cf. M.Du. vlugge, Low Ger. flügge). Originally an adj. meaning "having the feathers, fit to fly." As a verb, it is first attested in English 1560s. Related: Fledged; fledging.
fledgling: 1835 (adj.), 1846 as a noun meaning "young bird," from fledge + dim. suffix -ling. Of persons, from 1856.
flee: O.E. fleon "take flight" (contracted class II strong verb; past tense fleah, pp. flogen), from P.Gmc. *thleukhanan (cf. O.H.G. fliohan, O.N. flöja, Du. vlieden, Ger. fliehen, Goth. þliuhan "to flee"), of unknown origin. Not found outside Germanic. Weak pt./pp. fled emerged M.E., under influence of Scandinavian. O.E. had a trans. form, geflieman "put to flight," which came in handy in the Anglo-Saxon Chronicle. Related: Fleeing.
fleece (n.): O.E. fleos, from W.Gmc. *flusaz (cf. M.Du. vluus, M.H.G. vlius, Ger. Vlies), probably from PIE *plus- (cf. L. pluma "feather, down," Lith. plunksna "feather"). The verb is 1530s in the literal sense of "to strip a sheep of fleece" and 1570s in the figurative meaning "to cheat, swindle." Related: Fleeced; fleecing.
fleecy: 1560s, from fleece + -y (2).
fleer (v.): c.1400, perhaps from Scand. (cf. dial. Norw. flira, dial. Dan. flire "to grin, titter").
fleet (n.): O.E. fleot "ship, floating vessel," from fleotan "to float," from P.Gmc. *fleut-, from PIE base *pleu- "to flow, run, swim." Sense of "naval force" is pre-1200. The O.E. word also meant "creek, inlet, flow of water," especially one into the Thames near Ludgate Hill, which lent its name to Fleet Street (home of newspaper and magazine houses, standing for "the English press" since 1882), Fleet prison, etc.
fleeting (adj.): early 13c., from O.E. fleotende "floating, drifting," later "flying, moving swiftly," from O.E. fleotan (see fleet (n.)).
Fleming: O.E. Flæming "native or inhabitant of Flanders," from P.Gmc. *Flam- (cf. M.L. Flamingus).
Flemish: early 14c., flemmysshe, probably from O.Fris. Flemische, or from Fleming + -ish.
flesh (n.): O.E. flæsc "flesh, meat," also "near kindred" (a sense now obsolete except in phrase flesh and blood), common W. and N.Gmc. (cf. O.Fris. flesk, M.L.G. vlees, Ger. Fleisch "flesh," O.N. flesk "pork, bacon"), of unknown origin, perhaps from P.Gmc. *flaiskoz-. Figurative use for "animal or physical nature of man" (O.E.), is from the Bible, especially Paul's use of Gk. sarx, which yielded sense of "sensual appetites" (c.1200). Flesh-wound is from 1670s; flesh-color, the hue of "Caucasian" skin, is first recorded 1610s, described as a tint composed of "a light pink with a little yellow" [O'Neill, "Dyeing," 1862]. An O.E. poetry-word for "body" was flæsc-hama, lit. "flesh-home."
flesh (v.): 1520s, "to render (a hunting animal) eager for prey by rewarding it with flesh from a kill," with figurative extensions, from flesh (n.). Meaning "to clothe or embody with flesh," with figurative extensions, is from 1660s.
fleshpot: lit. "pot in which flesh is boiled," hence "luxuries regarded with envy," especially in fleshpots of Egypt, from Exodus xvi.3:
"Whan we sat by ye Flesh pottes, and had bred ynough to eate." [Coverdale translation, 1535]
fleshy: "plump," mid-14c., from flesh + -y (2).
fletch: mid-17c., variant of fledge (v.); also see fletcher. Related: Fletched; fletching.
fletcher: "arrow-maker," early 14c. (as a surname attested from 1203), from O.Fr. flechier, from fleche "arrow," probably from Frankish *fliugica (cf. O.Low Ger. fliuca, M.Du. vliecke).
fleur-de-lis: also fleur de lis, mid-14c., from O.Fr., lit. "flower of the lily," especially borne as a heraldic device on the royal arms of France. Perhaps originally representing an iris, or the head of a scepter, or a weapon of some sort.
flew: p.t. of fly (v.1), q.v.
flex (v.): 1520s, probably a back formation from flexibility. Related: Flexed; flexing.
flexibility: 1610s, of physical things, from Fr. flexibilité, from L. flexibilitatem, from flexibilis (see flexible). Of immaterial things from 1783.
flexible: early 15c., from L. flexibilis "that may be bent, pliant," from flexus, pp. of flectere "to bend," of uncertain origin.
flexion: c.1600, from L. flexionem, noun of action from flectere "to bend."
flexor: 1610s, Mod.L., agent noun from flectere "to bend."
flextime: 1972, translating Ger. Gleitzeit "sliding time."
flibbertigibbet: 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.
flick (n.): mid-15c., probably imitative of a light blow with a whip. Earliest recorded use is in phrase not worth a flykke "useless." As slang for "film," it is first attested 1926, a back formation from flicker, from their flickering appearance. The verb is first recorded 1838; meaning "quick turn of the wrist" is from 1897, originally in cricket. Related: Flicked; flicking.
flicker (v.): O.E. flicorian "to flutter, flap quickly and lightly," originally of birds. Onomatopoeic of quick motion. Sense of "shine with a wavering light" is c.1600, but not common till 19c. Related: Flickered; flickering.
flicker (n.): "woodpecker," 1808 Amer.Eng., possibly echoic of bird's note, or from white spots on plumage that seem to flicker as it flits from tree to tree.
flier: see flyer.
flight (1): "flying," O.E. flyht "a flying, flight." from P.Gmc. *flukhtiz (cf. Low Ger. flugt, Ger. Flucht). Spelling altered from M.E. fliht c. 1385 (see fight), from root of *fleugan "to fly" (see fly (v.1)). Meaning "an instance of flight" is 1785, originally of ballooning. Meaning "series of stairs between landings" is from 1703.
flight (2): "fleeing," from M.E. fluht (c.1200), not found in O.E., but presumed to have existed. Related to O.E. fleon "flee" (see flee).
flighty: 1550s, "swift," later (1768) "fickle or frivolous," originally of skittish horses; from flight (1) + -y. Related: Flightiness.
flim-flam: also flimflam, 1530s (n.); 1650s (v.); a contemptuous echoic construction, perhaps connected to some unrecorded dial. word from Scandinavian (cf. O.N. flim "a lampoon").
flimsy: 1702, perhaps a metathesis of film "gauzy covering."
flinch: 1570s, from obsolete flecche "to bend, flinch," probably from O.Fr. flenchir "to bend," from Frank. *hlankjan (cf. M.H.G. linken, Ger. lenken "to bend, turn, lead"). Related: Flinched; flinching.
flinders: mid-15c., Scottish flendris, probably related to Norw. flindra "chip, splinter," or Du. flenter "fragment," ult. from the same PIE root that produced flint (q.v.).
fling (v.): c.1300, probably from O.N. flengja, of uncertain origin. The M.E. intransitive sense is preserved in phrase have a fling at "make a try." The noun sense of "period of indulgence on the eve of responsibilities" first attested 1827. Meaning "vigorous dance" (associated with the Scottish Highlands) is from 1806. An obsolete word for "streetwalker, harlot" was fling-stink (1679).
flint: O.E. flint "flint, rock," common Gmc. (cf. M.Du. vlint, O.H.G. flins, Dan. flint), from PIE *splind- "to split, cleave," from base *(s)plei- "to splice, split" (cf. Gk. plinthos "brick, tile," O.Ir. slind "brick"). Transferred senses were in O.E.
flintlock: 1680s as a type of musket-firing mechanism, from flint + lock (1).
flinty: “hard-hearted,” 1530s, from flint + -y (2).
flip (v.): 1590s, possibly imitative; or a contraction of fillip (q.v.), which also is held to be imitative. Sense of "get excited" is first recorded 1950; flip one's lid "lose one's head, go wild" is from 1951. For flip (adj.) "glib," see flippant. Related: Flipped. Flipping (adj.) as euphemism for fucking is British slang first recorded 1911 in D.H. Lawrence.
flip (n.): sailors' hot drink usually containing beer, brandy and sugar, 1695, from flip (v.), so called from notion of it being "whipped up" or beaten.
flip-flop: also flip flop, "thong sandal," by 1972, imitative of the sound of walking in them (flip-flap had been used in various echoic senses since 1520s); sense of "complete reversal of direction" dates from 1900.
flippancy: 1746, from flippant + -cy.
flippant: "displaying unbecoming levity," c.1600, apparently an extended form of flip (v.). Shortened form flip is attested from 1847. Related: Flippantly.
flipper: "limb used to swim with," 1822, from the verb. Sense of "rubber fin for underwater swimming" is from 1945. Slang meaning "the hand" dates from 1836. Related: Flippers.
flirt (v.): 1550s, originally "to turn up one's nose, sneer at," then "to rap or flick, as with the fingers" (1560s). The noun is first attested 1540s, with the meaning "stroke of wit." It's possible that the original word was imitative, along the lines of flip (v.), but there seems to be some influence from flit, such as in the flirt sense of "to move in short, quick flights," attested from 1580s. Meanwhile flirt had come to mean "a pert young hussey" [Johnson] by 1560s, and Shakespeare has flirt-gill (i.e. Jill) "a woman of light or loose behavior," while flirtgig was a 17c. Yorkshire dialect word for "a giddy, flighty girl." All or any of these could have fed into the main modern verbal sense of "play at courtship" (1777), which also could have grown naturally from the earlier meaning "to flit inconstantly from object to object" (1570s), perhaps influenced by O.Fr. fleureter "talk sweet nonsense," also "to touch a thing in passing," dim. of fleur "flower" and metaphoric of bees skimming from flower to flower. The noun meaning "person who flirts" is from 1732. The English word also is possibly related to E.Fris. flirt "a flick or light blow," and flirtje "a giddy girl." Related: Flirted; flirting.