Online Etymology Dictionary

flan (n.): "open tart," 1846, from French flan "custard tart, cheesecake," from Old French flaon (12c.), from Medieval Latin flado, probably a Germanic borrowing (cf. Frankish *flado, Old High German flado "offering cake," Middle High German vlade "a broad, thin cake," Dutch vla "baked custard"), from Proto-Germanic *flatho(n), akin to words for "flat" and probably from PIE root *plat- "to spread" (see place). Borrowed earlier as flawn (c.1300), from Old French.
Flanders: probably a compound of roots represented by Flemish vlakte "plain" + wanderen "to wander."
flaneur (n.): "loafer, idler," 1854, from French flâneur, from flâner "to stroll, loaf, saunter," probably from a Scandinavian source (cf. Norwegian flana, flanta "to gad about").
flange (n.): 1680s, of unknown origin, perhaps related to Old French flanche "flank, side," fem. of flanc (see flank (n.)).
flank (n.): late Old English flanc "fleshy part of the side," from Old French flanc, probably from Frankish *hlanca (cf. Old High German (h)lanca, Middle High German lanke "hip joint," German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn" (see link (n.)). The military sense is first attested 1540s, as is the verb. Related: Flanked; flanking.
flannel (n.): c.1500, probably from Welsh gwlanen "woolen cloth," from gwlan "wool," from Celtic *wlana, from PIE *wele- "wool."

The Welsh origin is not a universally accepted etymology, due to the sound changes involved; some (Barnhart, Gamillscheg) suggest the English word is from an Anglo-French diminutive of Old French flaine "a kind of coarse 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]. Modern French flanelle is a 17c. borrowing from English.
flap (n.): mid-14c., flappe "a blow, slap," probably imitative of the sound of striking. Meaning "something that hangs down" is first recorded 1520s. Sense of "motion or noise like a bird's wing" is 1774; meaning "disturbance, noisy tumult" is 1916, British slang.
flap (v.): early 14c., "dash about, shake;" later "strike, hit;" see flap (n.). Meaning "to swing loosely" is from 1520s. Related: Flapped; flapping.
flapdoodle (n.): 1833, originally "the stuff they feed fools on" [Marryat]; an arbitrary formation.
flapjack (n.): pre-1600, from flap + jack (n.), using the personal name in its generic object sense.
flapper (n.): "forward young woman," 1921 slang, from flap (v.), but the exact connection is disputed. Perhaps from flapper "young wild-duck or partridge" (1747), with reference to flapping wings while learning to fly, of which many late 19c. examples 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."

But other suggested sources are late 19c. northern English dialectal use 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 was also "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.): mid-16c., originally "spread out" (hair), of unknown origin, perhaps from Dutch vlederen. Related: Flared; flaring. The notion of "spreading out in display" is behind the notion of "spreading gradually outward" (1640s). Flare-up "a sudden burst" is from 1837.
flare (n.): "bright, unsteady light," 1814, from flare (v.), which led to the sense of "signal fire" (1883). Flares "flared trousers" is from 1964.
flash (v.): late 14c., from flasken (c.1300) "to dash or splash" (as water), probably imitative. Related: Flashed; flashing. Sense of "give off a sudden burst of light or flame" is 1540s. Flash flood is from 1940. Flash card is from 1923. Flash cube (remember those?) is from 1965.
flash (n.): 1560s, from flash (v.); originally of lightning. Meaning "first news report" is from 1857. Meaning "photographic lamp" is from 1913. The comic book character dates to 1940. Flash in the pan (1809) is from old-style guns, where the powder might ignite in the pan but fail to spark the main charge.
flashback (n.): 1903, in reference to fires in engines or furnaces, from flash + back (adj.). 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.): "male genital exhibitionist," 1960s (though meat-flasher in this sense was attested in 1890s); agent noun from flash (v.).
flashing (adj.): 1570s, of light; present participle adjective from flash (v.).
flashing (n.): "indecent exposure," 1896, verbal noun from flash (v.). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but it is of unknown origin and might be an unrelated word.
flashlight (n.): American English for what the British might call an electric torch; 1919, from flash + light (n.).
flashpoint (n.): also flash point, 1878, from flash (v.) + point (n.). Figurative use by 1955.
flashy (adj.): "showy, cheaply attractive," 1680s, from flash + -y (2). Earlier it meant "splashing" (1580s); "sparkling, giving off flashes" (c.1600). Related: Flashily; flashiness.
flask (n.): mid-14c., from Medieval Latin flasco "container, bottle," from Late Latin flasconem "bottle," perhaps from a Germanic source (cf. Old English flasce, Old High German flaska, Middle Dutch flasce, German Flasche "bottle"), and if so, perhaps originally meaning "a bottle plaited round, case bottle" (cf. Old High German flechtan "to weave," Old English fleohtan "to braid, plait"), from Proto-Germanic base *fleh- (see flax).

Another theory traces it to a metathesis of Latin vasculum. "The assumption that the word is of Teut. origin is chronologically legitimate, and presents no difficulty exc. the absence of any satisfactory etymology" [OED].
flat (adj.): early 14c., from Old Norse flatr, from Proto-Germanic *flataz (cf. Old Saxon flat "flat, shallow,: Old High German flaz "flat, level," Old English flet, Old High German flezzi "floor"), perhaps from PIE *plat- "to spread" (cf. Greek platys "broad, flat;" see plaice (n.)).

Sense of "prosaic, dull" is from 1570s; used of drink from c.1600; of musical notes from 1590s, because the tone is "lowered." Flat-out (adv.) "openly, directly" is from 1932; earlier it was a noun meaning "total failure" (1870, U.S. colloquial).
flat (n.): 1801, from Scottish flat "floor or story of a house," from Old English flet "a dwelling, floor, ground," from the same source as flat (adj.).
flat-footed (adj.): c.1600, "with flat feet;" meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial use it meant "straightforwardly, downright" (1828), from notion of "standing firmly."
flat-top (n.): "aircraft carrier," 1943, U.S. Navy, from flat (adj.) + top (n.). As a style of haircut, from 1956.
flatboat (n.): 1650s, from flat (adj.) + boat (n.).
flatfish (n.): 1710, from flat (adj.) + fish (n.).
flatline (v.): by 1998, from the flat line 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., from flat (adj.) + -ness.
flats (n.): "level tidal tract," 1540s, from flat (n.) "level piece of ground" (late 12c.), from flat (adj.).
flatten (v.): late 14c., "to prostrate oneself," also "to fall flat," from flat (adj.) + -en (1). Meaning "to make flat" is 1620s. Related: Flattened; flattening.
flatter (v.): early 13c., from Old French flater "to flatter" (13c.), originally "stroke with the hand, caress," from Frankish *flat "palm, flat of the hand" (see flat (adj.)). "[O]ne of many imitative verbs beginning with fl- and denoting unsteady or light, repeated movement" [Liberman]. Related: Flattered; flattering.
flatterer (n.): mid-14c., agent noun from flatter. Fem. form flatteress is attested from late 14c.-18c.
flattering (adj.): late 14c., "pleasing to the imagination," present participle adjective from flatter. Meaning "gratifying to self-esteem" is from 1757. Related: Flatteringly.
flattery (n.): early 14c., from Old French flaterie, from flater (see flatter).
flatulence (n.): 1711, from French flatulence, from flatulent (see flatulent). Flatulency is from 1650s.
flatulent (adj.): 1590s, from Middle French flatulent, from Modern Latin flatulentus, from Latin flatus "a blowing, a breaking wind," past participle of flare "to blow, puff," which is cognate with Old English blawan (see blow (v.1)).
flatus (n.): 1660s, from Latin flatus "a blowing," from flare "to blow" (see blow (v.1)).
flatware (n.): 1851, from flat (adj.) + 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. It looks French, but it corresponds to no known French word. Transitive sense is from 1827. Related: Flaunted; flaunting.
flautist (n.): 1860, from Italian flautista, from flauto "flute" (from Late Latin flauta; see flute (n.)) + Greek-derived suffix -ista.
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 flaour "smell, odor," from Vulgar Latin flator "odor," literally "that which blows," from Latin flator "blower," from flare "to blow, puff," which is cognate with Old English blawan (see blow (v.1)).

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- is perhaps from influence of savor.
flavor (v.): 1730s, from flavor (n.). Related: Flavored; flavoring.
flavorful (adj.): 1927, from flavor (n.) + -ful. Earlier flavorsome (1853), flavory (1727), flavorous (1690s).
flavoring (n.): 1845, "thing that gives flavor," verbal noun from flavor (v.). Middle English flauryng meant "perfume."
flavour: chiefly British English spelling of flavor; for spelling, see -or. Related: Flavourful; flavouring.