Online Etymology Dictionary

flagellate (v.): "to whip, scourge," 1620s, from Latin flagellatus, past participle of flagellare "to scourge, lash" (see flagellum). Related: Flagellated; flagellating. An earlier verb for this in English was flagellen (mid-15c.; see flail (v.)).
flagellate (adj.): 1851, from flagellum + -ate (1).
flagellation (n.): early 15c., "the scourging of Christ," from Old French flagellacion "scourging, flogging," or directly from Latin flagellationem (nominative flagellatio) "a scourging," noun of action from past participle stem of flagellare "to scourge, lash" (see flagellum). In a general sense from 1520s.
flagellum (n.): "long, lash-like appendage," 1837, from Latin flagellum "whip, scourge," also figurative, diminutive of flagrum "a whip," from PIE root *bhlag- "to strike" (source also of Latin flagitium "shameful act, passionate deed, disgraceful thing," flagitare "to demand importunately;" Old Norse blakra "to flutter with the wings," blekkja "to impose upon;" Lithuanian blaškau "to and fro").
flageolet (n.): flute-like instrument, 1650s, from French flageolet, diminutive of Old French flajol, from Provençal flajol, which is of unknown origin.
flagitious (adj.): "shamefully wicked, criminal," late 14c., from Old French flagicieus or directly from Latin flagitiosus "shameful, disgraceful, infamous," from flagitium "shameful act, passionate deed, disgraceful thing," related to flagrum "a whip, scourge, lash," and flagitare "to demand importunately," all from PIE root *bhlag- "to strike" (see flagellum). Related: Flagitiously; flagitiousness.
flagman (n.): also flag-man, "signaler," 1832, from flag (n.1) + man (n.). Earlier it meant "admiral" (1660s).
flagon (n.): "large bottle for wine or liquor," mid-15c., from Middle French flacon, Old French flascon "small bottle, flask" (14c.), from Late Latin flasconem (nominative flasco) "bottle" (see flask).
flagpole (n.): also flag-pole, 1782, from flag (n.1) + pole (n.1). Flagpole-sitting as a craze is attested from 1927.
flagrance (n.): "glaring shamefulness," 1610s, from French flagrance or directly from Latin flagrantia "a glow, ardor, a burning desire," noun of quality from flagrantem "burning, blazing, glowing" (see flagrant). Related: Flagrancy (1590s).
flagrant (adj.): c. 1500, "resplendent" (obsolete), from Latin flagrantem (nominative flagrans) "burning, blazing, glowing," figuratively "glowing with passion, eager, vehement," present participle of flagrare "to burn, blaze, glow" from Proto-Italic *flagro- "burning" (source also of Oscan flagio-, an epithet of Iuppiter), corresponding to PIE *bhleg-ro-, from *bhleg- "to shine, flash, burn" (source also of Greek phlegein "to burn, scorch," Latin fulgere "to shine"), from root *bhel- (1) "to shine, flash, burn" (see bleach (v.)). Sense of "glaringly offensive, scandalous" (rarely used of persons) first recorded 1706, probably from common legalese phrase flagrante delicto "while the crime is being committed, red-handed," literally "with the crime still blazing." Related: Flagrantly.
flagship (n.): also flag-ship, 1670s, a warship bearing the flag of an admiral, vice-admiral, or rear-admiral, from flag (n.) + ship (n.). Properly, at sea, a flag is the banner by which an admiral is distinguished from the other ships in his squadron, other banners being ensigns, pendants, standards, etc. Figurative use by 1933.
flagstaff (n.): 1610s, from flag (n.) + staff (n.). The settlement in Arizona, U.S., said to have been so called for a July 4, 1876, celebration in which a large flag was flown from a tall tree.
flagstone (n.): "any rock which splits easily into flags," 1730, from flag (n.2) "flat, split stone" + stone (n.).
Flaherty: surname, Irish Flaithbheartach, literally "Bright-Ruler."
flail (n.): implement for threshing grain, c. 1100, perhaps from an unrecorded Old English *flegel, which, if it existed, probably is from West Germanic *flagil (source also of Middle Dutch and Low German vlegel, Old High German flegel, German flegel), a West Germanic borrowing of Late Latin flagellum "winnowing tool, flail," in classical Latin "a whip" (see flagellum).
flail (v.): mid-15c., "to whip, scourge," from flail (n.). Sense of "to move like a flail" is from 1873. Related: Flailed; flailing.
flair (n.): mid-14c., "an odor," from Old French flaire "odor or scent," especially in hunting, "fragrance, sense of smell," from flairier "to give off an odor; stink; smell sweetly" (Modern French flairer), from Vulgar Latin *flagrare, dissimilated from Latin fragrare "emit (a sweet) odor" (see fragrant). Sense of "special aptitude" is American English, 1925, probably from hunting and the notion of a hound's ability to track scent.
flak (n.): 1938, "anti-aircraft gun," from German Flak, condensed from Fliegerabwehrkanone, literally "pilot warding-off cannon." Sense of "anti-aircraft fire" is from 1940; metaphoric sense of "criticism" is c. 1963 in American English. Flak jacket is by 1956.
flake (n.): "thin, flat piece of snow; a particle," early 14c., also flauke, flagge, which is of uncertain origin, possibly from Old English *flacca "flakes of snow," or from cognate Old Norse flak "loose or torn piece" (related to Old Norse fla "to skin;" see flay); or perhaps from Proto-Germanic *flago- (source also of Middle Dutch vlac, Dutch vlak "flat, level," Middle High German vlach, German Flocke "flake"); from PIE *plak- (1) "to be flat" (see placenta). From late 14c. as "a speck, a spot."
flake (v.): early 15c., flaken, (of snow) "to fall in flakes," from flake (n.). Transitive meaning "break or peel off in flakes" is from 1620s; intransitive sense of "to come off in flakes" is from 1759. . Related: Flaked; flaking.
flaky (adj.): 1570s, "consisting of flakes," from flake + -y (2). Meaning "eccentric, crazy" first recorded 1959, said to be American English baseball slang, but probably from earlier druggie slang flake "cocaine" (1920s). Flake (n.) "eccentric person" is a 1968 back-formation from it. Related: Flakiness.
The term 'flake' needs explanation. It's an insider's word, used throughout baseball, usually as an adjective; someone is considered 'flaky.' It does not mean anything so crude as 'crazy,' but it's well beyond 'screwball' and far off to the side of 'eccentric.' ["New York Times," April 26, 1964]
flam (n.): 1630s, "sham story, fabrication," also as a verb, "to deceive by flattery;" see flim-flam.
flambe (adj.): 1869, of certain types of porcelain, 1914 as a term in cookery, from French flambé, past participle of flamber "to singe, blaze" (16c.), from Old French flambe "a flame" (see flamboyant). Middle English had flame (v.) in cookery sense "baste (a roast) with hot grease, to baste; to glaze (pastry)."
flambeau (n.): also flambeaux, 1630s, "flaming torch," from French flambeau (14c.), from flambe "flame" (see flamboyant). By 1883 as "a large, decorative candlestick."
flamboyance (n.): 1849, from flamboyant + -ance. Related: Flamboyancy (1846).
flamboyant (adj.): 1832, originally in reference to a 15c.-16c. architectural style with wavy, flame-like curves, from French flamboyant "flaming, wavy," present participle of flamboyer "to flame," from Old French flamboiier "to flame, flare, blaze, glow, shine" (12c.), from flambe "a flame, flame of love," from flamble, variant of flamme, from Latin flammula "little flame" (see flame (n.)). Extended sense of "showy, ornate" is from 1879. Related: Flamboyantly.
flame (n.): Middle English flaume, also flaumbe, flambe, flame, flamme, mid-14c., "a flame;" late 14c., "a flaming mass, a fire; fire in general, fire as an element;" also figurative, in reference to the "heat" or "fire" of emotions, from Anglo-French flaume, flaumbe "a flame" (Old French flambe, 10c.), from Latin flammula "small flame," diminutive of flamma "flame, blazing fire," from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn" (see bleach (v.)).

The meaning "a sweetheart, object of one's passion" is attested from 1640s; the figurative sense of "burning passion" was in Middle English, and the nouns in Old French and Latin also meant "fire of love, flame of passion," and, in Latin "beloved object." The Australian flame-tree is from 1857, so called for its red flowers.
flame (v.): Middle English flaumen, also flaumben, flomben, flamben, flamen, flammen, c. 1300 (implied in flaming "to shine (like fire), gleam, sparkle like flames;" mid-14c. as "emit flames, be afire, to blaze," from Anglo-French flaumer, flaumber (Old French flamber) "burn, be on fire, be alight" (intransitive), from flamme "a flame" (see flame (n.)).

Transitive meaning "to burn, set on fire" is from 1580s. Meaning "break out in violence of passion" is from 1540s; the sense of "unleash invective on a computer network" is from 1980s. Related: Flamed; flaming. To flame out, in reference to jet engines, is from 1950.
flame-thrower (n.): also Flamethrower, 1917, translating German flammenwerfer (1915). See flame (n.) + throw (v.).
flamen (n.): "ancient Roman priest," 1530s, from Latin flamen "a priest of one deity," which is of unknown origin, perhaps from PIE root *bhlad- "to worship" (source also of Gothic blotan, Old English blotan "to sacrifice"). Also used from early 14c., in imitation of Geoffrey of Monmouth, in reference to ancient pre-Christian British priests. Related: Flamineous.
The old connection of flamen with Skt. brahman- is highly problematic, and has been dismissed by Schrijver. As WH surmise, the ending -en points to an archaism, probably a n[euter] noun "sacrificial act" which changed its semantics to 'priest'; for a similar shift, cf. augur "bird-observer" .... The only viable comparanda are found in [Germanic], but they show root-final (or suffixal) *-d~. [de Vaan]
flamenco (n.): 1882, from Spanish flamenco, first used of Gypsy dancing in Andalusia. The word in Spanish meant "a Fleming, native of Flanders" (Dutch Vlaming) and also "flamingo." Speculation are varied and colorful about the connection between the bird, the people, and the gypsy dance of Andalusia.

Spain ruled Flanders for many years in 16c., and King Carlos I brought with him to Madrid an entire Flemish court. One etymology suggests the dance was so called from the bright costumes and energetic movements, which the Spanish associated with Flanders; another is that Spaniards, especially Andalusians, like to name things by their opposites, and because the Flemish were tall and blond and the gypsies short and dark, the gypsies were called "Flemish;" others hold that flamenco was the general Spanish word for all foreigners, gypsies included; or that Flemish noblemen, bored with court life, took to slumming among the gypsies. Compare Gypsy.
flamer (n.): 1590s, agent noun from flame (v.). Figurative sense "glaringly conspicuous person" is from 1809. For homosexual slang sense, see flaming.
flaming (adj.): late 14c., "flame-like in appearance;" c. 1400, "on fire," present participle adjective from flame (v.). Meaning "of bright or gaudy colors" is from mid-15c. As an intensifying adjective, late 19c. Meaning "glaringly homosexual" is homosexual slang, 1970s (along with flamer (n.) "conspicuously homosexual man"); but flamer "glaringly conspicuous person or thing" (1809) and flaming "glaringly conspicuous" (1781) are much earlier in a general sense, both originally with reference to "wenches." Related: Flamingly.
flamingo (n.): long-legged, long-necked brightly colored pink bird of the tropical Americas, 1560s, from Portuguese flamengo, Spanish flamengo, literally "flame-colored" (compare Greek phoinikopteros "flamingo," literally "red-feathered"), from Provençal flamenc, from flama "flame" (see flame (n.)) + Germanic suffix -enc "-ing, belonging to." Perhaps accommodated to words for Fleming (see flamenco).
flammable (adj.): 1813, from stem of Latin flammare "to set on fire" (from flamma; see flame (n.)) + -able. In modern (20c.) use, a way to distinguish from the ambiguity of inflammable.
flan (n.): "open tart," 1846, from French flan "custard tart, cheesecake," from Old French flaon "flat-cake, tart, flan" (12c.), from Medieval Latin flado (10c.), which probably is from Frankish *flado or another Germanic source (compare Old High German flado "offering cake," Middle High German vlade "a broad, thin cake," Dutch vla "baked custard"), from Proto-Germanic *flatho(n) "flat cake," probably from PIE root *plat- "to spread" (see plaice (n.)). Borrowed earlier as flawn (c. 1300), from Old French.
Flanders: from a source akin to Dutch Vlaanderen probably a compound of roots represented by Flemish vlakte "plain" + wanderen "to wander."
flaneur (n.): "habitual loafer, idle man about town," 1854, from French flâneur, from flâner "to stroll, loaf, saunter," probably from a Scandinavian source (compare Old Norse flana "to wander aimlessly," Norwegian flana, flanta "to gad about"), perhaps from PIE *pele- (2) "flat, to spread." Related: flânerie.
flange (n.): 1680s, "a widening or branching out," of unknown origin, perhaps related to Old French flanche "flank, hip, side," fem. of flanc (see flank (n.)). Meaning "projecting rim, etc., used for strength or guidance" is from 1735. As a verb from 1820.
flank (n.): late Old English flanc "flank, fleshy part of the side," from Old French flanc "hip, side," from Frankish or another Germanic source, from Proto-Germanic *hlanca- (source also of Old High German (h)lanca, Middle High German lanke "hip joint," German lenken "to bend, turn aside;" Old English hlanc "loose and empty, slender, flaccid;" Old Norse hlykkr "a bend, noose, loop"), from PIE root *kleng- "to bend, turn" (see link (n.)). Showing characteristic change of Germanic hl- to Romanic fl-. The military sense is first attested 1540s. Meaning "side" of anything is by 1620s. As an adjective, "pertaining to the flank or side," 1660s. Related: Flanked; flanking.
flank (v.): 1590s (military), "to guard the flank," also, "to menace the flank, fire sideways upon," from flank (n.). Meaning "stand or be placed at the side of" is from 1650s. Related: Flanked; flanking.
flanker (n.): 1550s, from flank (n.).
flannel (n.): "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.): 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.): 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.): 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.): 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.): 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.): 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.