Online Etymology Dictionary

flare (n.): "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.): "a sudden burst," 1827 of an argument; 1858 of light, from verbal phrase; see flare (v.) + up (adv.). It seems to have had some vogue as a street expression in London in the 1830s.
Flare up! flare up! is all the cry, in every square and street --
No other sound salutes your ear, whoe'er you chance to meet
Where'er you ride, or walk, or sit, or breakfast, dine, or sup,
They welcome you or quiz you with "Flare up, my boy! flare up!"

[Fraser's Magazine," April 1834]
flash (v.): 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): 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 (1704 literal, 1705 figurative) 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): "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.): 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.): also flashpoint, "temperature at which vapor will ignite momentarily," 1869, from flash (v.) + point (n.). Figurative use by 1955. Slightly earlier as flashing-point (1867).
flashback (n.): 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.): 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.): 1540s, of light; present participle adjective from flash (v.).
flashing (n.): 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.): 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 (flash-lamp in this sense is by 1890). From 1905 as as a handheld, pocket-sized electric illumination device, the American English word for what the British might call an electric torch.
flashy (adj.): "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.): mid-14c., from Medieval Latin flasco "container, bottle," from Late Latin flasconem (nominative flasco) "bottle," which is 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.): 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- (source also of 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; of women's bosoms by 1864. 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.): 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.): 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.): c. 1600, "to lay flat;" 1670s in music, from flat (adj.). Related: Flatted; flatting.
flat-boat (n.): also flat-boat, 1650s, from flat (adj.) + boat (n.).
flat-car (n.): 1839 in railroading, from flat (adj.) + car (n.).
flat-footed (adj.): 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.): "iron for smoothing," 1810, from flat (adj.) + iron (n.). Applied to triangular or wedge-shaped buildings from 1862.
flat-top (n.): 1943, "aircraft carrier," U.S. Navy, from flat (adj.) + top (n.). As a style of haircut, from 1956.
flatfish (n.): also flat-fish, 1710, from flat (adj.) + fish (n.). So called from the shape.
flatland (n.): 1735, from flat (adj.) + land (n.). Edwin Abbott's popular book about an imaginary two-dimensional world was published in 1884.
flatline (v.): "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.): 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.): mid-15c., "state or quality of being flat," from flat (adj.) + -ness.
flats (n.): "level tidal tract," 1540s, from flat (n.) in the Middle English "level piece of ground" sense.
flatten (v.): 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.): 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.): 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.): 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.): early 14c., "dishonest praise, coaxing speech," from Old French flaterie "flattery, cajolery" (Modern French flatterie), from flater "to flatter" (see flatter).
flatulence (n.): 1711, from French flatulence, from flatulent (see flatulent). Flatulency is from 1650s.
flatulent (adj.): "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.): 1660s, "wind in the bowels," from Latin flatus "a blowing," from flare "to blow" (see blow (v.1)).
flatware (n.): 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.): 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.): 1620s, "act or habit of flaunting," from flaunt (v.).
flautist (n.): 1827, from Italian flautista, from flauto "flute" (from Late Latin flauta; see flute (n.)) + Greek-derived suffix -ista.
Flavian (adj.): 1590s, pertaining to the three Roman empoerors who reigned 69-96 C.E., the dynasty of (Flavius) Vespasian; see Flavius
Flavius: masc. proper name, from Latin Flavius, a Roman gens name, related to flavus "golden-yellow, blond" (see blue), and probably originally meaning "yellow-haired."
flavor (n.): c. 1300, "a smell, odor" (usually a pleasing one), from Old French flaor "smell, odor; action of smelling, sense of smell," probably from Vulgar Latin flator "odor," literally "that which blows," in classical Latin "blower," from flare "to blow, puff," which is cognate with Old English blawan (see blow (v.1)).

"Not common before Milton's time" [Century Dictionary], and it is not clear what exactly Milton meant when he used it. The same Vulgar Latin source produced Old Italian fiatore "a bad odor." Sense of "taste, savor" is 1690s, perhaps 1670s; originally "the element in taste which depends on the sense of smell." The -v- in the English word is euphonic or perhaps from influence of savor. Flavor-of-the-month is from 1946 (originally of ice cream).
flavor (v.): 1540s, "communicate a distinctive quality to," from flavor (n.). Meaning "add a flavoring substance to" is from 1740. Earliest use was now-obsolete sense of "to smell" (early 15c.). Related: Flavored; flavoring.
flavorful (adj.): 1904, from flavor (n.) + -ful. Earlier flavorsome (1853), flavory (1727), flavorous (1690s).
flavoring (n.): "thing that gives flavor," 1845, originally in cookery, verbal noun from flavor (v.). Middle English flauryng meant "perfume."
flavorless (adj.): 1730, from flavor (n.) + -less. Related: Flavorlessly; flavorlessness.
flavour: chiefly British English spelling of flavor; for spelling, see -or. Related: Flavoured; flavourful; flavouring.
flaw (n.): early 14c., "a flake" (of snow), also in Middle English "a spark of fire; a splinter," from Old Norse flaga "stone slab, layer of stone," perhaps used here in a wider sense (see flag (n.2)). Old English had floh stanes, but the Middle English form suggests a Scandinavian origin. "The close resemblance in sense between flaw and flake is noteworthy" [OED]. Sense of "defect, fault" first recorded 1580s, first of character, later (c. 1600) of material things; probably via notion of a "fragment" broken off.