Online Etymology Dictionary

flight (n.2): "act of fleeing," from Middle English fluht (c.1200), not found in Old English, but presumed to have existed. Related to Old English fleon "flee" (see flee), and cognate with Old Saxon fluht, Old Frisian flecht "act of fleeing," Dutch vlucht, Old High German fluht, German Flucht, Old Nprse flotti, Gothic þlauhs.
flightless (adj.): 1875, from flight (n.1) + -less.
flighty (adj.): 1550s, "swift," later (1768) "fickle or frivolous," originally of skittish horses; from flight (n.1) + -y (2). Related: Flightiness.
flim-flam (n.): also flimflam, 1530s, a contemptuous echoic construction, perhaps connected to some unrecorded dialectal word from Scandinavian (cf. Old Norse flim "a lampoon"). From 1650s as a verb.
flimsy (adj.): 1702, of unknown origin, perhaps a metathesis of film (n.) "gauzy covering" + -y (2). Related: Flimsily; flimsiness.
flinch (v.): 1570s, from obsolete flecche "to bend, flinch," probably from Old French flenchir "to bend," probably from Frankish *hlankjan or some other Germanic source (cf. Middle High German linken, German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn" (see link (n.)). Related: Flinched; flinching. As a noun, from 1817.
flinders (n.): 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, probably from or related to Old Norse flengja "to flog," of uncertain origin. The Middle English intransitive sense is that suggested by phrase have a fling at "make a try." An obsolete word for "streetwalker, harlot" was fling-stink (1670s). Related: Flung; flinging.
fling (n.): "attempt, attack," early 14c.; see fling (v.). Sense of "period of indulgence on the eve of responsibilities" first attested 1827. Meaning "vigorous dance" (associated with the Scottish Highlands) is from 1806.
flint (n.): Old English flint "flint, rock," common Germanic (cf. Middle Dutch vlint, Old High German flins, Danish flint), from PIE *splind- "to split, cleave," from root *(s)plei- "to splice, split" (cf. Greek plinthos "brick, tile," Old Irish slind "brick"). Transferred senses were in Old English.
flintlock (n.): 1680s as a type of musket-firing mechanism, from flint + lock (n.1).
flintstone (n.): early 14c., from flint + stone (n.).
flinty (adj.): "hard-hearted," 1530s, from flint + -y (2). Literal sense of "resembling flint" is from 1640s. Related: Flintily; flintiness.
flip (v.): 1590s (1520s in flip-flop), imitative or else 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 1950. For flip (adj.) "glib," see flippant. Meaning "to flip a coin" (to decide something) is by 1879. As a noun by 1690s. 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.): 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-flop (n.): also flip flop, "thong sandal," by 1972, imitative of the sound of walking in them (flip-flap had been used in various echoic senses, mostly echoic, since 1520s); sense of "complete reversal of direction" dates from 1900.
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]
flippancy (n.): 1746, from flippant + -cy.
flippant (adj.): c.1600, "talkative;" 1670s, "displaying unbecoming levity," apparently an extended form of flip (v.). 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, 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, from the verb, 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 (n.) 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 Old French fleureter "talk sweet nonsense," also "to touch a thing in passing," diminutive 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 East Frisian flirt "a flick or light blow," and flirtje "a giddy girl." French flirter "to flirt" is a 19c. borrowing from English. Related: Flirted; flirting.
flirtation (n.): 1718, noun of action from flirt (v.).
flirtatious (adj.): 1834, from flirtation + -ous. Related: Flirtatiously; flirtatiousness.
flirty (adj.): 1840, from flirt (v.) + -y (2). Related: Flirtiness.
flit (v.): c.1200, flutten "convey, move, take, carry away, go away," perhaps from Old Norse flytja "to remove, bring."
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, from 1835.
flitch (n.): "side of bacon," Middle English flicche (early 13c.), from Old English flicce, related to Old Norse flikki, Middle Low German vlicke "piece of flesh." Not immediately connected to flesh (n.), but perhaps from the same PIE root. A flitch was presented every year at 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 as far back as mid-14c.
flitter (v.): 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 flutter." Related: Flittered; flittering. As a noun, from 1892.
flitty (adj.): 1640s, from flit + -y (2). Related: Flittiness.
flivver (n.): "cheap car," especially "Model-T Ford," 1910, of unknown origin.
float (v.): late Old English flotian "to float" (class II strong verb; past tense fleat, past participle floten), from Proto-Germanic *flutojanan (cf. Old Norse flota, Middle Dutch vloten), from PIE root *pleu- "to flow" (see pluvial). Of motion through air, from 1630s. Related: Floated; floating.
float (n.): early 12c., "state of floating" (Old English flot meant "body of water"), from float (v.). 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.): "dead body found in water," 1890, U.S. slang, agent noun from float (v.).
floc (n.): 1921, diminutive of flocculus (see flocculate).
floccinaucinihilipilification (n.): "action or habit of estimating as worthless," 1741, a combination of four Latin words (flocci, nauci, nihili, pilifi) all signifying "at a small price" or "for nothing," which were listed together in a rule of the well-known Eton Latin Grammar. 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."
flocculate (v.): 1877, from flocculus (1799, from Modern Latin diminutive of Latin floccus "flock of wool") + -ate. Related: Flocculated; flocculating.
flocculation (n.): 1885, from flocculate + -ion.
flocculent (adj.): "resembling wool," 1800, from Latin floccus "lock of hair, flock of wool" + -ulent. Related: Floculence.
flock (n.1): Old English flocc "a group of persons, company, troop," related to Old Norse flokkr "crowd, troop, band," Middle Low German vlocke "crowd, flock (of sheep);" not found in other Germanic languages; perhaps related to folc "people," but the metathesis would have been unusual for Old English.

Extended c.1200 to "a number of animals of one kind moving or feeding together;" of domestic animals c.1300. Transferred to bodies of Christians, in relation to Christ or their local pastor, from mid-14c.
flock (n.2): "tuft of wool," mid-13c., probably from Old French floc, from Latin floccus "flock of wool, lock of hair."
flock (v.): "gather, congregate," c.1300, from flock (n.). Related: Flocked; flocking.
floe (n.): 1817, first used by Arctic explorers, probably from Norwegian flo "layer, slab," from Old Norse flo, related to first element in flagstone (q.v.). Earlier explorers used flake.
flog (v.): 1670s, slang, perhaps a schoolboy shortening of Latin flagellare "flagellate." Related: Flogged; flogging.
flood (n.): Old English flod "a flowing of water, flood, an overflowing of land by water, Noah's Flood; mass of water, river, sea, wave," from Proto-Germanic *flothuz (cf. Old Frisian flod, Old Norse floð, Middle Dutch vloet, Dutch vloed, German Flut, Gothic flodus), from PIE verbal stem *pleu- "flow, float" (see pluvial). Figurative use by mid-14c.
flood (v.): 1660s, from flood (n.). Related: Flooded; flooding.
floodgate (n.): early 13c. in the figurative sense (especially with reference to tears or rain); literal sense is mid-15c.; from flood (n.) + gate (n.).
floodplain (n.): also flood plain, 1873, from flood (n.) + plain (n.).
floor (v.): early 15c., "to furnish with a floor," from floor (n.). Sense of "puzzle, confound" is 1830, from notion of "knock down to the floor" (1640s). Related: Floored; flooring.
floor (n.): Old English flor "floor, pavement, ground, bottom (of a lake, etc.)," from Proto-Germanic *floruz "floor" (cf. Middle Dutch and Dutch vloer, Old Norse flor "floor," Middle High German vluor, German Flur "field, meadow"), from PIE *plaros "flat surface" (cf. Welsh llawr "ground"), enlarged from *pele- (2) "flat, to spread" (see plane (n.1)).

Meaning "level of a house" is from 1580s. The figurative sense in legislative assemblies (as opposed to the platform) is first recorded 1774. Spanish suelo "floor" is from Latin solum "bottom, ground, soil;" German Boden is cognate with English bottom. Floor plan attested from 1867.
flooring (n.): "materials of a floor," 1620s, verbal noun from floor (v.).
floozie (n.): also floozy, "woman of disreputable character," 1902, perhaps a variation of flossy "fancy, frilly" (1890s slang), with the notion of "fluffiness."