Etymology
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float (n.)

apparently an early Middle English merger of three related Old English nouns, flota "boat, fleet," flote "troop, flock," flot "body of water, sea;" all from the source of float (v.). The early senses were the now-mostly-obsolete ones of the Old English words: "state of floating" (early 12c.), "swimming" (mid-13c.); "a fleet of ships; a company or troop" (c. 1300); "a stream, river" (early 14c.). From c. 1300 as an attachment for buoyancy on a fishing line or net; early 14c. as "raft." Meaning "platform on wheels used for displays in parades, etc." is from 1888, probably from earlier sense of "flat-bottomed boat" (1550s). As a type of fountain drink, by 1915.

Float.—An ade upon the top of which is floated a layer of grape juice, ginger ale, or in some cases a disher of fruit sherbet or ice cream. In the latter case it would be known as a "sherbet float" or an "ice-cream float." ["The Dispenser's Formulary: Or, Soda Water Guide," New York, 1915]
Few soda water dispensers know what is meant by a "Float Ice Cream Soda." This is not strange since the term is a coined one. By a "float ice cream soda" is meant a soda with the ice cream floating on top, thus making a most inviting appearance and impressing the customer that you are liberal with your ice cream, when you are not really giving any more than the fellow that mixes his ice cream "out of sight." [The Spatula, Boston, July, 1908]
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float (v.)

late Old English flotian "to rest on the surface of water" (intransitive; class II strong verb; past tense fleat, past participle floten), from Proto-Germanic *flotan "to float" (source also of Old Norse flota, Middle Dutch vloten, Old High German flozzan, German flössen), from PIE *plud-, extended form of root *pleu- "to flow."

Meaning "drift about, hover passively" is from c. 1300. Transitive sense of "to lift up, cause to float" (of water, etc.) is from c. 1600; that of "set (something) afloat" is from 1778 (originally of financial operations). Of motion through air, from 1630s. Meaning "hover dimly before the eyes" is from 1775. Related: Floated; floating. A floating rib (by 1802) is so called because the anterior ends are not connected to the rest.

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floater (n.)
"one who or that which floats," 1717, agent noun from float (v.). From 1847 in political slang for an independent voter (but with suggestion of purchasability); 1859 as "one who frequently changes place of residence or employment." Meaning "dead body found in water" is 1890, U.S. slang.
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flotation (n.)
1765, from float (v.) + -ation. Spelling influenced by French flotaison (compare floatation).
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afloat (adv., adj.)
Old English aflote, contraction of prepositional phrase on flot; see a- (1) "to" + flot "body of water deep enough for a boat" (see float (n.)).
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flutter (v.)
Old English floterian "to flutter (of birds), to fly before, flicker, float to and fro, be tossed by waves," frequentative of flotian "to float" (see float (v.)). Meaning "throw (someone) into confusion" is from 1660s. Related: Fluttered; fluttering. As a noun, "quick, irregular motion," from 1640s; meaning "state of excitement" is 1740s. Flutterpate "flighty person" is from 1894.
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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 and cognate with float) + -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.
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*pleu- 
Proto-Indo-European root meaning "to flow."

It forms all or part of: fletcher; fledge; flee; fleet (adj.) "swift;" fleet (n.2) "group of ships under one command;" fleet (v.) "to float, drift; flow, run;" fleeting; flight (n.1) "act of flying;" flight (n.2) "act of fleeing;" flit; float; flood; flotsam; flotilla; flow; flue; flugelhorn; fluster; flutter; fly (v.1) "move through the air with wings;" fly (n.) "winged insect;" fowl; plover; Pluto; plutocracy; pluvial; pneumo-; pneumonia; pneumonic; pulmonary.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit plavate "navigates, swims;" Greek plynein "to wash," plein "to navigate," ploein "to float, swim," plotos "floating, navigable," pyelos "trough, basin;" Latin plovere "to rain," pluvius "rainy;" Armenian luanam "I wash;" Old English flowan "to flow;" Old Church Slavonic plovo "to flow, navigate;" Lithuanian pilu, pilti "to pour out," plauju, plauti "to swim, rinse."
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fleet (v.)
Old English fleotan "to float; drift; flow, run (as water); swim; sail (of a ship)," from Proto-Germanic *fleutan (source also of Old Frisian fliata, Old Saxon fliotan "to flow," Old High German fliozzan "to float, flow," German fliessen "to flow, run, trickle" (as water), Old Norse fliota "to float, flow"), from PIE root *pleu- "to flow."

Meaning "to glide away like a stream, vanish imperceptibly" is from c. 1200; hence "to fade, to vanish" (1570s). Related: Fleeted; fleeting.
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buoyant (adj.)
1570s, perhaps from Spanish boyante, present participle of boyar "to float," from boya "buoy," from Dutch boei (see buoy (n.)). Of personalities, etc., from c. 1748. Related: Buoyantly.
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