"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- "ready to fly" (cognates: Middle Dutch vlugge, Low German flügge), from PIE *pleuk- "to fly" (see fletcher). Meaning "bring up a bird" (until it can fly on its own) is from 1580s. Related: Fledged; fledging.