Online Etymology Dictionary

flop (v.): c.1600, probably a variant of flap with a duller, heavier sound. Sense of "fall or drop heavily" is 1836, that of "collapse, fail" is 1919; though the figurative noun sense of "a failure" is recorded from 1893. Related: Flopped; flopping.
flop (n.): 1823, in the literal sense, from flop (v.). Figurative use by 1893.
flophouse (n.): "cheap hotel," hobo slang, 1904, probably related to slang flop (v.) "lie down for sleep" (1907); see flop (v.) + house (n.).
In one of [Cincinnati's] slum districts stands the Silver Moon, a "flop house" (i.e., a house where the occupants are "flopped" out of their hanging bunks by letting down the ropes) .... ["McClure's" magazine, November 1904]
(But this explanation is not found in other early references.)
floppy (adj.): 1858, from flop + -y (2). Floppy disc attested from 1972 (short form floppy by 1974).
flora (n.): 1777, "the plant life of a region or epoch," from Latin Flora, Roman goddess of flowers, from flos (genitive floris) "flower," from *flo-s-, Italic suffixed form of PIE *bhle- "to blossom, flourish" (cf. Middle Irish blath, Welsh blawd "blossom, flower," Old English blowan "to flower, bloom"), extended form of *bhel- (3) "to thrive, bloom," possibly identical with *bhel- (2) "to blow, swell" (see bole). Used as the title of descriptive plant catalogues since 1640s, but popularized by Linnaeus in his 1745 study of Swedish plants, "Flora Suecica."
floral (adj.): 1640s, "pertaining to Flora," from French floral (16c.), from Latin floralis "of flowers" (see flora). Meaning "pertaining to flowers" is from 1753.
Florence: fem. proper name, from Latin Florentia, fem. of Florentius, literally "blooming," from florens (genitive florentis), present participle of florere "to flower" (see flourish).

The c.1700 "Dictionary of the Canting Crew" defines Florence as a slang word for "a Wench that is touz'd and ruffled." This was also the Italian city name (Roman Colonia Florentia, "flowering colony," either literal or figurative), which became Old Italian Fiorenze, in modern Italian Firenze.
Florentine (adj.): 1540s, literally "of or pertaining to the Italian city of Florence," from Latin Florentinus, from Florentia (see Florence). Earliest reference in English is to a type of textile fabric.
florescence (n.): 1793, from Modern Latin florescentia, from Latin florescentem (nominative florescens) "blooming," present participle of florescere "to begin to bloom," inceptive of florere "to blossom" (see flourish (v.)).
florescent (adj.): 1821, from Latin florescentem, present participle of florescere (see florescence).
floret (n.): c.1400, flourette, from Old French florete "little flower; cheap silk material," diminutive of flor "flower," from Latin flora (see flora). Botany sense is from 1670s.
floricide (n.): "one who destroys flowers," 1841, from Latin floris, genitive of flos "flower" (see flora) + -cide.
floriculture (n.): 1822, from Latin floris, genitive of flos "flower" (see flora) + -culture on analogy of agriculture. Related: Floricultural; floriculturist.
florid (adj.): 1640s, "strikingly beautiful," from French floride "flourishing," from Latin floridus "flowery, in bloom," from flos "flower" (see flora). Sense of "ruddy" is first recorded 1640s. Meaning "profusely adorned, as with flowers," is from 1650s. Related: Floridly.
Florida: U.S. state, formerly a Spanish colony, probably from Spanish Pascua florida, literally "flowering Easter," a Spanish name for Palm Sunday, because the peninsula was discovered on that day (March 20, 1513) by the expedition of Spanish explorer Ponce de León (1474-1521). From Latin floridus (see florid).
florin (n.): c.1300, from Old French florin, from Italian fiorino, from fiore "flower," from Latin florem "flower" (see flora). The 13c. gold Florentine coin was stamped on the obverse with the image of a lily, the symbol of the city. As the name of an English gold coin, from late 15c.
florist (n.): 1620s, formed on analogy of French fleuriste, from Latin floris, genitive of flos "flower" (see flora) + -ist.
floruit: used now mainly in sense of "period during which a historical person's life work was done," 1843, Latin, literally "he flourished," third person singular perf. indicative of florere (see flourish (v.)). Usually in abbreviation fl.
floss (n.): "rough silk," 1759, perhaps from French floche "tuft of wool" (16c.), from Old French floc "tuft, lock," from Latin floccus "tuft of wool." Or from an unrecorded Old English or Old Norse word from the root found in Dutch flos "plush" (17c.). Cf. the surname Flossmonger, attested 1314, which might represent a direct borrowing from Scandinavian or Low German. In "The Mill on the Floss" the word is the proper name of a fictitious river in the English Midlands.
flossy (adj.): "resembling floss," 1839, from floss + -y (2).
flotation (n.): 1850s, from float (v.) + -ation. Spelling influenced by French (cf. floatation).
flotilla (n.): 1711, "a small fleet," from Spanish flotilla, diminutive of flota "float," from flotar "to float," of Germanic origin (see float (v.)).
flotsam (n.): c.1600, from Anglo-French floteson, from Old French flotaison "a floating," from floter "to float" (of Germanic origin; see float) + -aison, from Latin -ation(em). Spelled flotsen till mid-19c. when it altered, perhaps under influence of many English words in -some.

In British law, flotsam are goods found floating on the sea as a consequence of a shipwreck or action of wind or waves; jetsam are things cast out of a ship in danger of being wrecked, and afterward washed ashore, or things cast ashore by the sailors. Whatever sinks is lagan. Figurative use for "odds and ends" attested by 1861.
flounce (v.): 1540s, "to dash, plunge, flop," perhaps from Scandinavian (cf. dialectal Swedish flunsa "to plunge," Norwegian flunsa "to hurry," but first record of these is 200 years later than the English word), said to be of imitative origin. Spelling likely influenced by bounce. Notions of "anger, impatience" began to adhere to the word 18c. Related: Flounced; flouncing. As a noun, from 1580s as a motion.
flounce (n.): "wide ruffle," 1713, from Middle English frounce "pleat, wrinkle, fold" (late 14c.), from Old French fronce "line, wrinkle; pucker, crease, fold," from Frankish *hrunkjan "to wrinkle," from Proto-Germanic *hrunk-. Influenced in form by flounce (v.).
flounder (v.): 1590s, perhaps an alteration of founder (q.v.), influenced by Dutch flodderen "to flop about," or native verbs in fl- expressing clumsy motion. Figurative use is from 1680s. Related: Floundered; floundering. As a noun derived from this sense, from 1867.
flounder (n.): flatfish, c.1300, from Anglo-French floundre, from Old North French flondre, from Old Norse flydhra; related to Middle Low German vlundere, Danish flynder; ultimately cognate with Greek platys "flat, wide, broad" (see plaice (n.)).
flour (n.): early 13c., flur "flower" (see flower (n.)); meaning "finer portion of ground grain" is mid-13c., from the notion of flour as the "finest part" of meal (cf. French fleur de farine), as distinguished from the coarser parts (meal). Spelled flower until flour became the accepted form c.1830 to end confusion.
flour (v.): "to sprinkle with flour," 1650s, from flour (n.). Related: Floured; flouring.
flourish (v.): c.1300, "to blossom, grow," from Old French floriss-, stem of florir "blossom, flower, bloom, flourish," from Latin florere "to bloom, blossom, flower," figuratively "to flourish, be prosperous," from flos "a flower" (see flora).

Metaphoric sense of "thrive" is mid-14c. Meaning "to brandish (a weapon)" first attested late 14c. Related: Flourished; flourishing.
flourish (n.): c.1500, "a blossom," from flourish (v.). Meaning "ostentatious waving of a weapon" is from 1550s; that of "literary or rhetorical embellishment" is from c.1600.
flout (v.): 1550s, perhaps a special use of Middle English flowten "to play the flute" (cf. Middle Dutch fluyten "to play the flute," also "to jeer"). Related: Flouted; flouting.
flow (v.): Old English flowan "to flow, stream, issue; become liquid, melt; abound, overflow" (class VII strong verb; past tense fleow, past participle flowen), from Proto-Germanic *flo- (cf. Middle Dutch vloyen, Dutch vloeien "to flow," Old Norse floa "to deluge," Old High German flouwen "to rinse, wash"), probably from PIE *pleu- "flow, float" (see pluvial). The weak form predominated from 14c., but strong past participle flown is occasionally attested through 18c. Related: Flowed; flowing.
flow (n.): mid-15c., "action of flowing," from flow (v.). Meaning "amount that flows" is from 1807. Flow chart attested from 1920.
flower (n.): c.1200, from Old French flor "flower, blossom; heyday, prime; fine flour; elite; innocence, virginity" (Modern French fleur), from Latin florem (nominative flos) "flower" (source of Italian fiore, Spanish flor; see flora).

Modern spelling is 14c. Ousted Old English cognate blostm (see blossom (n.)). Also used from 13c. in sense of "finest part or product of anything" and from c.1300 in the sense of "virginity." Flower children "gentle hippies" is from 1967.
flower (v.): c.1200, "be vigorous, prosper, thrive," from flower (n.). Of a plant or bud, "to blossom," c.1300. Related: Flowered; flowering.
flowerpot (n.): 1590s, from flower (n.) + pot (n.1).
flowery (adj.): mid-14c., from flower (n.) + -y (2). Related: Floweriness.
flown: past participle of fly (v.).
flu (n.): 1839, flue, shortening of influenza. Spelling flu attested from 1893. For choice of middle syllable, not a common method of shortening in English, Weekley compares tec for detective, scrip for subscription.
flub (v.): 1924, American English, perhaps suggested by fluff, flop, etc. Related: Flubbed; flubbing. As a noun, by 1952.
fluctuant (adj.): 1550s, from Latin fluctuantem (nominative fluctuans), present participle of fluctuare "to move in waves" (see fluctuation).
fluctuate (v.): 1630s, from Latin fluctuatus, past participle of fluctuare "to undulate" (see fluctuation). Related: Fluctuated; fluctuates; fluctuating.
fluctuation (n.): mid-15c., from Middle French fluctuation (12c.) or directly from Latin fluctuationem (nominative fluctuatio) "a wavering, vacillation," noun of action from past participle stem of fluctuare "to undulate, to move in waves," from fluctus "wave, billow, surge," from past participle of fluere "to flow" (see fluent).
flue (n.): "smoke channel in a chimney," 1580s, perhaps related to 15c. word meaning "mouthpiece of a hunting horn," or perhaps from Old English flowan "to flow," and/or Old French fluie "stream."
fluency (n.): 1620s, "abundance," later "smooth and easy flow" (1630s), from fluent + -cy. Replaced earlier fluence (c.1600).
fluent (adj.): 1580s, "flowing freely" (of water, also of speech), from Latin fluentem (nominative fluens) "lax, relaxed," figuratively "flowing, fluent," present participle of fluere "to flow, stream, run, melt," from PIE *bhleugw-, extended form of *bhleu- "to swell, well up, overflow" (cf. Latin flumen "river;" Greek phluein "to boil over, bubble up," phlein "to abound"), an extension of root *bhel- (2) "to blow, inflate, swell;" see bole. Used interchangeably with fluid in Elizabethan times. Related: Fluently.
fluff (n.): "light, feathery stuff," 1790, apparently a variant of floow "wooly substance, down, nap" (1580s), perhaps from Flemish vluwe, from French velu "shaggy, hairy," from Latin vellus "fleece," or Latin villus "tuft of hair" (see velvet). OED suggests fluff as "an imitative modification" of floow, "imitating the action of puffing away some light substance." Slang bit of fluff "young woman" is from 1903. The marshmallow confection Fluff dates to c.1920 in Massachusetts, U.S.
fluff (v.): "to shake into a soft mass," 1875, from fluff (n.). Meaning "make a mistake" is from 1884, originally in theater slang. Related: Fluffed; fluffing.
fluffer (n.): "track sweeper on the London underground," by 1956.