Etymology
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fledge (v.)
"to acquire feathers," 1560s, from Old English adjective *-flycge (Kentish -flecge; in unfligge "featherless," glossing Latin implumes) "having the feathers developed, fit to fly," from Proto-Germanic *flugja- "feather" (source also of Middle Dutch vlugge, Low German flügge), from PIE *pluk- "to fly," extended form of root *pleu- "to flow." Meaning "bring up a bird" (until it can fly on its own) is from 1580s. Related: Fledged; fledging.
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fledged (adj.)
"furnished with feathers," 1570s (in full-fledged), thus "developed, matured, able to fly;" past-participle adjective from fledge (v.).
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unfledged (adj.)
c. 1600, of persons, "immature, not experienced," from un- (1) "not" + past participle of fledge (v.). Literal sense of "not yet covered in feathers" is recorded from 1610s.
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fledgling 
also fledgeling, 1830, "untried" (adj.), in Tennyson; 1846 as a noun meaning "young bird" (one newly fledged); from fledge + diminutive suffix -ling. Of persons, from 1856.
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fletch (v.)
"fit feathers to" (an arrow), 1650s, variant of fledge (v.) in sense "fit (an arrow) with feathers, altered by influence of fletcher. Related: Fletched; fletching.
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fly (adj.)
slang, "clever, alert, wide awake," by 1811, perhaps from fly (n.) on the notion of the insect being hard to catch. Other theories, however, trace it to fledge or flash. Slang use in 1990s might be a revival or a reinvention.
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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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full-fledged (adj.)
1570s, literal; 1883 in figurative sense; see full (adj.) + fledged.
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