Online Etymology Dictionary

flocculate (v.): "gather in flocculent masses," 1845 (flocculated), from flocculus (1799), from Modern Latin, a diminutive of Latin floccus "flock of wool" (see flock (n.2)) + -ate (2). Related: Flocculating.
flocculation (n.): "the union of small particles into granular aggregates," 1875, from flocculate + -ion.
flocculent (adj.): "resembling wool, fleecy," 1800, from Latin floccus "lock of hair, flock of wool" (see flock (n.2)) + -ulent. Related: Flocculence.
flock (n.1): Old English flocc "a group of persons, company, troop," related to Old Norse flokkr "crowd, troop, band," Middle Low German vlocke "crowd, flock (of sheep);" of unknown origin, not found in other Germanic languages; perhaps related to folc "people," but the metathesis would have been unusual for Old English.

In Old English of humans only; extended c. 1200 to "a number of animals of one kind moving or feeding together;" of domestic animals c. 1300. The special reference to birds is recent (19c.). Transferred to bodies of Christians, in relation to Christ or their pastor, from mid-14c.
flock (n.2): "tuft of wool," mid-13c., also found in continental Germanic and Scandinavian, all probably from Old French floc, from Latin floccus "flock of wool, lock of hair."
flock (v.): c. 1300 "gather, congregate" (intransitive), from flock (n.1). Related: Flocked; flocking.
floe (n.): 1817, first used by Arctic explorers, probably from Norwegian flo "layer, slab," from Old Norse flo, from Proto-Germanic *floho-, from PIE *plak- (1) "to be flat," extended form of root *pele- (2) "flat, to spread" (see plane (n.1)). Related to first element in flagstone. Earlier explorers used flake. Floe-rat was a seal-hunter's name for the ringed seal (1880).
flog (v.): 1670s, slang, of uncertain origin. Perhaps a schoolboy shortening of Latin flagellare "flagellate" (see flagellum); Century Dictionary suggests perhaps from a Low German word "of homely use, of which the early traces have disappeared." OED finds it presumably onomatopoeic. Figurative use from 1800. Related: Flogged; flogging.
flogging (n.): 1793, verbal noun from flog (v.). Earlier in the same sense was floggation (1680s).
flood (n.): Old English flōd "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave," from Proto-Germanic *floduz "flowing water, deluge" (source also of Old Frisian flod, Old Norse floð, Middle Dutch vloet, Dutch vloed, German Flut, Gothic flodus), from the source of Old English flowan, from PIE verbal root *pleu- "to flow, float, swim" (see pluvial). In early modern English often floud. Figurative use, "a great quantity, a sudden abundance," by mid-14c.
flood (v.): 1660s, "to overflow" (transitive), from flood (n.). Intransitive sense "to rise in a flood" is from 1755. Related: Flooded; flooding.
flood-gate (n.): early 13c. in the figurative sense "opportunity for a great venting" (especially with reference to tears or rain); literal sense is mid-15c. (gate designed to let water in or keep it out as desired, especially the lower gate of a lock); from flood (n.) + gate (n.).
flood-plain (n.): 1844, from flood (n.) + plain (n.).
flood-tide (n.): 1719, from flood (n.) + tide (n.).
floodlight (n.): also flood-light, 1924, from flood (n.) + light (n.). Related: Floodlit.
floor (n.): Old English flor "floor, pavement, ground, bottom (of a lake, etc.)," from Proto-Germanic *floruz "floor" (source also of Middle Dutch and Dutch vloer, Old Norse flor "floor," Middle High German vluor "floor, flooring," German Flur "field, meadow"), from PIE *plaros "flat surface" (source also of Welsh llawr "ground"), enlarged from *pele- (2) "flat, broad; to spread out" (see plane (n.1)).

Meaning "level of a house" is from 1580s. The figurative sense in legislative assemblies (1774) is in reference to the "floor" where members sit and from which they speak (as opposed to the platform). Spanish suelo "floor" is from Latin solum "bottom, ground, soil;" German Boden is cognate with English bottom (n.). Floor-plan is attested from 1794; floor-board from 1787, floor-lamp from 1886, floor-length (adj.) of dresses is from 1910. The retail store's floor-walker is attested from 1862.
floor (v.): early 15c., "to furnish with a floor," from floor (n.). Sense of "puzzle, confound" is from 1830, a figurative use, from earlier sense of "knock down to the floor" (1640s). In mid-19c. English university slang, it meant "do thoroughly and successfully" (1852). Related: Floored; flooring.
flooring (n.): "materials of a floor," 1620s, verbal noun from floor (v.).
floozie (n.): also floozy, "woman of disreputable character," 1902, perhaps a variation of flossy "fancy, frilly" (1890s slang), with the notion of "fluffiness." The c. 1700 "Dictionary of the Canting Crew" defines Florence as a slang word for "a Wench that is touz'd and ruffled."
flop (v.): c. 1600, "to flap," 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, "act of flopping; any action that produces the sound 'flop;' the sound itself," from flop (v.). Figurative sense of "a failure; that which is a failure" is by 1893, from the notion of a sudden break-down or collapse. Extended form flopperoo attested from 1936. The Fosbury flop high-jumping technique (1968) is so called in reference to U.S. athlete Dick Fosbury (b.1947), who used it to win the 1968 Olympic gold medal.
flophouse (n.): "cheap hotel," hobo slang, 1904, probably related to slang flop (v.) "lie down for sleep" (1907); see flop (v.) + house (n.). The explanation below is not found in other early references.
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]
floppy (adj.): 1858, "inclined to flop" [OED], from flop + -y (2). Floppy disc attested from 1972 (short form floppy by 1974).
flora (n.): c. 1500, "Roman goddess of flowers;" 1777, "the plant life of a region or epoch," from Latin Flora, "goddess of flowers," from flos (accusative florem, genitive floris) "flower," from *flo-s-, Italic suffixed form of PIE *bhle- "to blossom, flourish" (source also of Middle Irish blath, Welsh blawd "blossom, flower," Old English blowan "to flower, bloom"), extended form of *bhel- (3) "to thrive, bloom," which is possibly identical with or derived from *bhel- (2) "to blow, swell" (see bole).

Her festival, the Floralia, was April 28 to May 2 and featured "comic theatrical representations" and "excessive drinking" [Century Dictionary]. The French Revolutionary calendar had a month Floréal (April 20-May 20). Used as the title of systematically descriptive plant catalogues since 1640s, but popularized by Linnaeus in his landmark 1745 study of Swedish plants, "Flora Suecica."
floral (adj.): 1640s, "pertaining to Flora," from French floral (16c.), from Latin floralis "pertaining to Flora; of flowers" (see flora). Meaning "pertaining to flowers" in English is from 1753. Related: Florally.
Florence: chief city of Tuscany, also a fem. proper name, both from Latin Florentia, fem. of Florentius, literally "blooming," from florens (genitive florentis), present participle of florere "to flower" (see flourish). The city name is from Roman Colonia Florentia, "flowering colony," either literal or figurative, and became Old Italian Fiorenze, modern Italian Firenze.
Florentine (adj.): 1540s, literally "of or pertaining to the Italian city of Florence," from Latin Florentinus, from Florentia, the Roman name of the city (see Florence). Earliest reference in English is to a type of textile fabric. As a noun from 1590s.
florescence (n.): "process of flowering," 1764, 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.): "bursting into bloom," 1784, from Latin florescentem, present participle of florescere "to begin to bloom" (see florescence).
floret (n.): c. 1400, flourette, "a little flower, a bud," from Old French florete "little flower," also the name of a cheap silk material, diminutive of flor "flower, blossom" (see flower (n.)). Botany sense "small flower in a cluster" is from 1670s.
floricide (n.): "one who destroys flowers," 1842, from Latin floris, genitive of flos "flower" (see flora) + -cide.
[S]urely there is cruelty and gross selfishness in cutting down for our own fleeting gratification that which would have ministered to the enjoyments of all for weeks or months. Frankly do I confess that I dislike a wanton floricide. He has robbed the world of a pleasure; he has blotted out a word from God's earth-written poetry. ["New Monthly Magazine" 1847]
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 "highly decorated, 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, and so named 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 "flowery, in bloom" (see florid). Related: Floridian (1580s as a noun, in reference to the natives; 1819 as an adjective).
florin (n.): type of coin, 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.): "one who cultivates flowers," especially "one who raises flowers for sale," 1620s, formed on analogy of French fleuriste, from Latin floris, genitive of flos "flower" (see flora) + -ist. Related: Floristry.
floruit: "period during which a historical person's life work was done," 1843, Latin, literally "he flourished," third person singular perfect indicative of florere "to flourish, to bloom" (see flourish (v.)). Usually in abbreviation fl. The third person singular present subjunctive of the verb, floreat, sometimes is attached to proper names "to indicate the hope that the named person, institution, etc., may prosper" [OED].
florulent (adj.): "flowery," 1590s, from Latin florentulus, from flor-, stem of flos "flower" (see flower (n.)).
floss (n.): "rough silk," 1759, of uncertain origin, perhaps from French floche "tuft of wool" (16c.), from Old French floc "tuft, lock," from Latin floccus "tuft of wool" (see flock (n.2)). Or from a dialectal survival of an unrecorded Old English or Old Norse word from the root of fleece (n.). Compare 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. Meaning "fine silk thread" is from 1871, short for floss silk (1759). Dental floss is from 1872; the verb floss in reference to use of it is from 1909. Related: Flossed; flossing.
flossy (adj.): "resembling floss," 1817, from floss (n.) + -y (2).
flotation (n.): 1765, from float (v.) + -ation. Spelling influenced by French flotaison (compare floatation).
flotilla (n.): "a small fleet," 1711, from Spanish flotilla, diminutive of flota "a fleet," from flotar "to float," which is said to be from Old French floter "to float, set afloat," which is from a word in Frankish or some other Germanic language corresponding to Old English flood (n.). Compare Old Norse floti "raft; fleet." Sometimes also "a fleet of small ships."
flotsam (n.): c. 1600, from Anglo-French floteson, from Old French flotaison "a floating" (Modern French flottaison), from floter "to float, set afloat" (of Germanic origin; see flotilla) + -aison, from Latin -ation(em). Spelled flotsen in English till mid-19c. when it altered, perhaps under influence of many English words in -some. Folk-etymologized in dialect as floatsome.

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. Flotsam and jetsam figuratively for "odds and ends" is attested by 1861.
flounce (v.): 1540s, "to dash, plunge, flop," perhaps from Scandinavian (compare dialectal Swedish flunsa "to plunge," Norwegian flunsa "to hurry, work hurriedly," 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 in reference to a sudden fling or turn of the body; by mid-18c. especially as expressing impatience or disdain.
flounce (n.): "deep ruffle on the skirt of a dress," 1713, from Middle English frounce "pleat, wrinkle, fold" (late 14c.), from Old French fronce "line, wrinkle; pucker, crease, fold," from Frankish *hrunkjan "to wrinkle," perhaps ultimately from the same Germanic source as shrink (v.). Influenced in form by flounce (v.). The verb meaning "arrange in flounces" is from 1711.
flounder (v.): "struggle awkwardly and impotently," especially when hampered somehow, 1590s, of uncertain origin, 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, "act of struggling," by 1867.
flounder (n.): "flatfish," c. 1300, from Anglo-French floundre, Old North French flondre, from Old Norse flydhra, from Proto-Germanic *flunthrjo (source also of Middle Low German vlundere, Danish flynder, Old Swedish flundra "flatfish"), suffixed and nasalized form of PIE *plat- "to spread" (cognate: Greek platys "flat, wide, broad;" see plaice (n.)).
flour (n.): "finer portion of ground grain," mid-13c., from flower (n.), and maintaining its older spelling, on the notion of flour as the "finest part" of meal, perhaps as the flower is the finest part of the plant or the fairest plant of the field (compare French fleur de farine), as distinguished from the coarser parts (meal (n.2)). Old French flor also meant both "a flower, blossom" and "meal, fine flour." The English word also was spelled flower until flour became the accepted form c. 1830 to end confusion. Flour-knave "miller's helper" is from c. 1300.
flour (v.): "to sprinkle with flour," 1650s, from flour (n.). Meaning "convert (wheat) into flour" is from 1828. Related: Floured; flouring.
flourish (v.): c. 1300, "to blossom, grow" (intransitive), from Old French floriss-, stem of florir "to blossom, flower, bloom; prosper, 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. in English. Transitive meaning "brandish (a weapon), hold in the hand and wave about" is from late 14c. Related: Flourished; flourishing.