flannel (n.) Look up flannel at Dictionary.com
"warm, loosely woven woolen stuff," c.1300, flaunneol, probably related to Middle English flanen "sackcloth" (c.1400); by Skeat and others traced to Welsh gwlanen "woolen cloth," from gwlan "wool," from Celtic *wlana, from PIE *wele- (1) "wool" (see wool). "As flannel was already in the 16th c. a well-known production of Wales, a Welsh origin for the word seems antecedently likely" [OED].

The Welsh origin is not a universally accepted etymology, due to the sound changes involved; Barnhart, Gamillscheg, Diez suggest the English word is from an Anglo-French diminutive of Old French flaine "a kind of coarse wool." Modern French flanelle is a 17c. borrowing from English.
flap (n.) Look up flap at Dictionary.com
mid-14c., flappe "a blow, slap, buffet," probably imitative of the sound of striking. Sense of "device for slapping or striking" is from early 15c. Meaning "something that hangs down" is first recorded 1520s, probably from flap (v.). Sense of "motion or noise like a bird's wing" is 1774; meaning "disturbance, noisy tumult" is 1916, British slang.
flap (v.) Look up flap at Dictionary.com
early 14c., "dash about, shake, beat (the wings);" later "strike, hit" (mid-14c.); probably ultimately imitative. Meaning "to swing about loosely" is from 1520s. Related: Flapped; flapping.
flapdoodle (n.) Look up flapdoodle at Dictionary.com
1833, originally "the stuff they feed fools on" [Marryat]; probably an arbitrary formation from elements meant to sound ridiculous, perhaps with allusions to flap "a stroke, blow" and doodle "fool, simpleton."
flapjack (n.) Look up flapjack at Dictionary.com
pre-1600, from flap (v.) + jack (n.), using the personal name in its "generic object" sense. So called from the process of baking it by flipping and catching it in the griddle when done on one side.
flapper (n.) Look up flapper at Dictionary.com
1560s, "one who or that which flaps," agent noun from flap (v.). Sense of "forward young woman" is 1921 slang, but the exact connection is disputed. Perhaps from flapper "young wild-duck or partridge" (1747), with reference to flapping wings while learning to fly, many late 19c. examples of which are listed in Wright's "English Dialect Dictionary" (1900), including one that defines it as "A young partridge unable to fly. Applied in joke to a girl of the bread-and-butter age."

Other suggested sources include a late 19c. northern English dialectal use of the word for "teen-age girl" (on notion of one with the hair not yet put up), or an earlier meaning "prostitute" (1889), which is perhaps from dialectal flap "young woman of loose character" (1610s). Any or all of these might have converged in the 1920s sense. Wright also has flappy, of persons, "wild, unsteady, flighty," with the note that it also was "Applied to a person's character, as 'a flappy lass,'" and further on he lists flappy sket (n.) "an immoral woman." In Britain the word took on political tones in reference to the debate over voting rights.
"Flapper" is the popular press catch-word for an adult woman worker, aged twenty-one to thirty, when it is a question of giving her the vote under the same conditions as men of the same age. ["Punch," Nov. 30, 1927]
flare (v.) Look up flare at Dictionary.com
1540s, "spread out" (hair), of unknown origin, perhaps from Scandinavian or from Dutch vlederen. Meaning "shine out with a sudden light" is from 1630s. The notion of "spreading out in display" is behind the notion of "spreading gradually outward" (1640s). Related: Flared; flaring.
flare (n.) Look up flare at Dictionary.com
"a giving off of a bright, unsteady light," 1814, from flare (v.). This led to the sense of "signal fire" (1883). The astronomy sense is from 1937. Meaning "a gradual widening or spreading" is from 1910; hence flares "flared trousers" (1964).
flare-up (n.) Look up flare-up at Dictionary.com
"a sudden burst," 1837, of an argument; 1858 of light, from verbal phrase; see flare (v.) + up (adv.).
flash (v.) Look up flash at Dictionary.com
Middle English flashen, flasken (c.1200), "sprinkle or splash (water, powder, etc.); to gush forth;" probably at least partly imitative (compare splash, dash). from c.1400, of birds, "to dart or flit" also, of fire, "burst into flames." Some of the extended senses perhaps are from Scandinavian. Meanings "burst suddenly into view" (intransitive) and "emit or send forth suddenly" (transitive) are from 1580s. the Sense of "expose the genitals" is recorded by 1846. Related: Flashed; flashing. Flash card is from 1923.
flash (n.1) Look up flash at Dictionary.com
1560s, "sudden burst of flame or light," from flash (v.); originally of lightning. Figuratively (of wit, laughter, anger, etc.) from c.1600. Meaning "period occupied by a flash, very short time" is from 1620s. Sense of "superficial brilliancy" is from 1670s. Meaning "first news report" is from 1857. The comic book character dates to 1940. Meaning "photographic lamp" is from 1913. Flash cube (remember those?) is from 1965.

Flash in the pan (1809) is from old-style firearms, where the powder might ignite in the pan but fail to spark the main charge; hence figurative sense "brilliant outburst followed by failure."
flash (n.2) Look up flash at Dictionary.com
"sudden rush of water," 1660s, earlier "watery place or marsh, a swamp" (c.1400; in place names from c.1300), of uncertain origin or connection to flash (n.1); perhaps from Old French flache, from Middle Dutch vlacke. Flash flood is from 1940.
flash (adj.) Look up flash at Dictionary.com
from flash (v.) in various and unconnected senses, often slang; sense of "of or associated with thieves, prostitutes, etc." is from c.1700. That of "vulgar, showy" is from 1785 (it is older in flashy). That of "expert, smart" is from 1812.
flash-point (n.) Look up flash-point at Dictionary.com
also flashpoint, flashing-point, "temperature at which vapor will ignite momentarily," 1878, from flash (v.) + point (n.). Figurative use by 1955.
flashback (n.) Look up flashback at Dictionary.com
also flash-back, 1903 in reference to fires in engines or furnaces, from verbal phrase (1902), from flash (v.) + back (adv.). Movie plot device sense is from 1916. The hallucinogenic drug sense is attested in psychological literature from 1970, which means probably hippies were using it a few years before.
flasher (n.) Look up flasher at Dictionary.com
1680s, "something that emits light in flashes," agent noun from flash (v.). Meaning "male genital exhibitionist" is from 1960s (meat-flasher in this sense was attested in 1890s and flash (v.) in the sense "expose the genitals" is recorded by 1846). Johnson (1755) has it also in the sense "one who makes a show of more wit than he possesses."
flashing (adj.) Look up flashing at Dictionary.com
1540s, of light; present participle adjective from flash (v.).
flashing (n.) Look up flashing at Dictionary.com
1791, "act of creating an artificial flood," verbal noun from flash (v.); also compare flash (n.2)). Meaning "indecent exposure" is by 1968 (see flasher). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but the sense connection is unclear and it might be an unrelated word.
flashlight (n.) Look up flashlight at Dictionary.com
also flash-light, 1886, "on-and-off signal light in a light-house, etc.," from flash (v.) + light (n.). As the word for a photographer's light-emitting preparation, 1892. As the American English word for what the British might call an electric torch; 1919.
flashy (adj.) Look up flashy at Dictionary.com
"showy, cheaply attractive," 1680s, from flash (n.1) + -y (2). Earlier it meant "splashing" (1580s); "sparkling, giving off flashes" (c.1600), but those senses have become rare. Related: Flashily; flashiness.
flask (n.) Look up flask at Dictionary.com
mid-14c., from Medieval Latin flasco "container, bottle," from Late Latin flasconem (nominative flasco) "bottle," of uncertain origin. a word common to Germanic and Romanic, but it is unclear whether the Latin or Germanic word is the original (or whether both might have got it from the Celts). Those who support a Germanic origin compare Old English flasce "flask, bottle" (which would have become modern English *flash), Old High German flaska, Middle Dutch flasce, German Flasche "bottle." IF it is Germanic, the original sense might be "bottle plaited round, case bottle" (compare Old High German flechtan "to weave," Old English fleohtan "to braid, plait"), from Proto-Germanic base *fleh- (see flax).

Another theory traces the Late Latin word to a metathesis of Latin vasculum. "The assumption that the word is of Teut[onic] origin is chronologically legitimate, and presents no difficulty exc[ept] the absence of any satisfactory etymology" [OED]. The similar words in Finnish and Slavic are held to be from Germanic.
flat (adj.) Look up flat at Dictionary.com
c.1300, "stretched out (on a surface), prostrate, lying the whole length on the ground;" mid-14c., "level, all in one plane; even, smooth;" of a roof, "low-pitched," from Old Norse flatr "flat," from Proto-Germanic *flata- (cognates: Old Saxon flat "flat, shallow," Old High German flaz "flat, level," Old English flet (for which see flat (n.)), Old High German flezzi "floor"), from PIE *plat- "to spread" (source of Greek platys "broad, flat;" see plaice (n.)). From c.1400 as "without curvature or projection."

Sense of "prosaic, dull" is from 1570s, on the notion of "featureless, lacking contrast." Used of drink from c.1600; of musical notes from 1590s, because the tone is "lower" than a given or intended pitch. Flat tire or flat tyre is from 1908. Flat-screen (adj.) in reference to television is from 1969 as a potential technology. Flat-earth (adj.) in reference to refusal to accept evidence of a global earth, is from 1876.
flat (n.) Look up flat at Dictionary.com
1801, "a story of a house," from Scottish flat "floor or story of a house," from Old English flett "a dwelling; floor, ground," from the same source as flat (adj.). Meaning "floor or part of a floor set up as an apartment" is from 1824. Directly from flat (adj.) come the senses "level ground near water" (late 13c.); "a flat surface, the flat part of anything" (1374), and "low shoe" (1834).
flat (adv.) Look up flat at Dictionary.com
1550s, "absolutely, downright;" 1570s, "plainly, positively," from flat (adj.). Flat-out (adv.) "openly, directly" is from 1932, originally in motor racing, picked up in World War II by the airmen; earlier it was a noun meaning "total failure" (1870, U.S. colloquial).
flat (v.) Look up flat at Dictionary.com
c.1600, "to lay flat;" 1670s in music, from flat (adj.). Related: Flatted; flatting.
flat-boat (n.) Look up flat-boat at Dictionary.com
also flat-boat, 1650s, from flat (adj.) + boat (n.).
flat-car (n.) Look up flat-car at Dictionary.com
1839 in railroading, from flat (adj.) + car (n.).
flat-footed (adj.) Look up flat-footed at Dictionary.com
c.1600, "with flat feet;" see flat (adj.) + foot (n.). Meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial adverbial use it meant "straightforwardly, downright, resolute" (1828), from notion of "standing firmly."
flat-iron (n.) Look up flat-iron at Dictionary.com
"iron for smoothing," 1810, from flat (adj.) + iron (n.). Applied to triangular or wedge-shaped buildings from 1862.
flat-top (n.) Look up flat-top at Dictionary.com
1943, "aircraft carrier," U.S. Navy, from flat (adj.) + top (n.). As a style of haircut, from 1956.
flatfish (n.) Look up flatfish at Dictionary.com
also flat-fish, 1710, from flat (adj.) + fish (n.). So called from the shape.
flatland (n.) Look up flatland at Dictionary.com
1735, from flat (adj.) + land (n.). Edwin Abbott's popular book about an imaginary two-dimensional world was published in 1884.
flatline (v.) Look up flatline at Dictionary.com
"give no indication of life, cease to function," by 1998, from the flat (adj.) line (n.) on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.
flatly (adv.) Look up flatly at Dictionary.com
early 15c. in a literal sense, from flat (adj.) + -ly (2). Meaning "in a plain manner" is from 1560s; sense of "in a dull manner" is from 1640s.
flatness (n.) Look up flatness at Dictionary.com
mid-15c., "state or quality of being flat," from flat (adj.) + -ness.
flats (n.) Look up flats at Dictionary.com
"level tidal tract," 1540s, from flat (n.) in the Middle English "level piece of ground" sense.
flatten (v.) Look up flatten at Dictionary.com
late 14c., "to prostrate oneself," also "to fall flat," from flat (adj.) + -en (1). Transitive meaning "to make flat" is 1620s. Related: Flattened; flattening.
flatter (v.) Look up flatter at Dictionary.com
c.1200, flateren, flaterien, "seek to please or gratify (someone) by undue praise, praise insincerely, beguile with pleasing words," from Old French flater "to deceive; caress, fondle; prostrate, throw, fling (to the ground)" (13c.), probably from a Germanic source, perhaps from Proto-Germanic *flata- "flat" (see flat (adj.)).

"Of somewhat doubtful etymology" [OED]. Liberman calls it "one of many imitative verbs beginning with fl- and denoting unsteady or light, repeated movement" (for example flicker, flutter). If it is related to flat the notion could be either "caress with the flat of the hand, stroke, pet," or "throw oneself flat on the ground" (in fawning adoration). The -er ending is unusual for an English verb from French; perhaps it is by influence of shimmer, flicker, etc., or from flattery.

Meaning "give a pleasing but false impression to" is from late 14c. Sense of "show (something) to best advantage" is from 1580s, originally of portraits. Related: Flattered; flattering.
flatterer (n.) Look up flatterer at Dictionary.com
mid-14c., agent noun from flatter. An old contemptuous term for one was flattercap (1680s). Fem. form flatteress is attested from late 14c.-18c.
flattering (adj.) Look up flattering at Dictionary.com
late 14c., "pleasing to the imagination; dishonestly pleasing; having a false appearance of favorableness," present participle adjective from flatter. Meaning "gratifying to self-esteem" is from 1757. Related: Flatteringly.
flattery (n.) Look up flattery at Dictionary.com
early 14c., "dishonest praise, coaxing speech," from Old French flaterie "flattery, cajolery" (Modern French flatterie), from flater "to flatter" (see flatter).
flatulence (n.) Look up flatulence at Dictionary.com
1711, from French flatulence, from flatulent (see flatulent). Flatulency is from 1650s.
flatulent (adj.) Look up flatulent at Dictionary.com
"affected by digestive gas," 1590s, from Middle French flatulent (16c.), from Modern Latin flatulentus, from Latin flatus "a blowing, breathing, snorting; a breaking wind," past participle of flare "to blow, puff," which is cognate with Old English blawan (see blow (v.1)).
flatus (n.) Look up flatus at Dictionary.com
1660s, "wind in the bowels," from Latin flatus "a blowing," from flare "to blow" (see blow (v.1)).
flatware (n.) Look up flatware at Dictionary.com
1851, from flat (adj.), which was used from late 14c. of plates, dishes, saucers in a sense "shallow; smooth-surfaced" + ware (n.). Originally as distinguished from hollow ware; U.S. sense of "domestic cutlery" recorded by 1895.
flaunt (v.) Look up flaunt at Dictionary.com
1560s, "to display oneself in flashy clothes," of unknown origin. Perhaps a variant of flout or vaunt. Perhaps from Scandinavian, where the nearest form seems to be Swedish dialectal flankt "loosely, flutteringly," from flakka "to waver" (related to flag (v.1)). It looks French, but it corresponds to no known French word. Transitive sense, "flourish (something), show off, make an ostentatious or brazen display of" is from 1827. Related: Flaunted; flaunting; Flauntingly.
flaunt (n.) Look up flaunt at Dictionary.com
1620s, "act or habit of flaunting," from flaunt (v.).
flautist (n.) Look up flautist at Dictionary.com
1860, from Italian flautista, from flauto "flute" (from Late Latin flauta; see flute (n.)) + Greek-derived suffix -ista.
Flavian (adj.) Look up Flavian at Dictionary.com
1590s, pertaining to the three Roman empoerors who reigned 69-96 C.E., the dynasty of (Flavius) Vespasian; see Flavius
Flavius Look up Flavius at Dictionary.com
masc. proper name, from Latin Flavius, a Roman gens name, related to flavus "golden-yellow, blond" (see blue), and probably originally meaning "yellow-haired."