Etymology
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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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fleet (n.)

Old English fleot "a ship, a raft, a floating vessel," also, collectively, "means of sea travel; boats generally," from fleotan "to float, swim," from Proto-Germanic *fleutanan(source also of Old Saxon fliotan, Old Frisian fliata, Old Norse fljta, Old High German fliozzan, Middle Dutch vlieten "to flow"), from PIE *pleud-, extended form of root *pleu- "to flow."

The sense of "naval force, group of ships under one command" is in late Old English. The more usual Old English word was flota "a ship," also "a fleet; a sailor." The fleet for "the navy" is attested by 1712. The Old English word also meant "estuary, inlet, flow of water," especially the one into the Thames near Ludgate Hill, which lent its name to Fleet Street (home of newspaper and magazine houses, hence its use metonymically for "the English press" since at least 1882) and Fleet prison (long used for debtors).

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fleece (n.)
"wool coat of a sheep," Old English fleos, flies "fleece, wool, fur, sealskin," from West Germanic *flusaz (source also of Middle Dutch vluus, Dutch vlies, Middle High German vlius, German Vlies), which is of uncertain origin; probably from PIE *pleus- "to pluck," also "a feather, fleece" (source also of Latin pluma "feather, down," Lithuanian plunksna "feather").
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fleeting (adj.)
early 13c., "fickle, shifting, unstable," from Old English fleotende "floating, drifting," later "flying, moving swiftly," from present participle of fleotan "to float, drift, flow" (see fleet (v.)). Meaning "existing only briefly" is from 1560s. Related: Fleetingly.
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fleecy (adj.)
1560s, "wooly," from fleece (n.) + -y (2). From 1630s as "resembling fleece" in any sense (originally by Milton, of clouds).
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fleer (v.)
"grin mockingly," c. 1400, perhaps from Scandinavian (compare dialectal Norwegian flira "giggle, laugh at nothing," dialectal Danish flire "to grin, sneer, titter"). Transitive sense from 1620s. Related: Fleered; fleering; fleeringly. As a noun from c. 1600.
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fleet (adj.)
"swift," 1520s, but probably older than the record; apparently from or cognate with Old Norse fliotr "swift," from Proto-Germanic *fleutaz, from PIE *pleud-, extended form of root *pleu- "to flow." Related: Fleetness.
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fleece (v.)
1530s in the literal sense of "to strip (a sheep) of fleece," from fleece (n.). From 1570s in the figurative meaning "to cheat, swindle, strip of money." Related: Fleeced; fleecer; fleecing.
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fugue (n.)

type of musical composition, 1590s, fuge, from Italian fuga, literally "flight," also "ardor," from Latin fuga "a running away, act of fleeing," from fugere "to flee" (see fugitive (adj.)). Current English spelling (1660s) is from the French version of the Italian word.

A Fugue is a composition founded upon one subject, announced at first in one part alone, and subsequently imitated by all the other parts in turn, according to certain general principles to be hereafter explained. The name is derived from the Latin word fuga, a flight, from the idea that one part starts on its course alone, and that those which enter later are pursuing it. ["Fugue," Ebenezer Prout, 1891]
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flea (n.)

Old English flea "flea," from Proto-Germanic *flauhaz (source also of Old Norse flo, Middle Dutch vlo, German Floh), perhaps related to Old English fleon "to flee," with a notion of "the jumping parasite," but more likely from PIE *plou- "flea" (source also of Latin pulex, Greek psylla; see Pulex).

Chaucer's plural is fleen. Flea-bag is from 1839 as low slang for "a bed;" by 1942 in British slang as "a dog." Flea-collar is from 1953. Flea-pit (1937) is an old colloquial name for a movie-house, or, as OED puts it, "an allegedly verminous place of public assembly." Flea-circus is from 1886. 

"A man named 'Mueller' put on the first trained-flea circus in America at the old Stone and Austin museum in Boston nearly forty years ago. Another German named 'Auvershleg' had the first traveling flea circus in this country thirty years ago. In addition to fairs and museums, I get as high as $25 for a private exhibition." ["Professor" William Heckler, quoted in Popular Mechanics, February 1928. Printed at the top of his programs were "Every action is visible to the naked eye" and "No danger of desertion."]
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