fly (n.) Look up fly at Dictionary.com
O.E. fleoge, from P.Gmc. *fleugjon (cf. O.S. fleiga, O.N. fluga, M.Du. vlieghe, Ger. Fliege “fly); lit. “the flying (insect)” (cf. O.E. fleogende “flying”), from same source as fly(v.1). Originally “any winged insect” (hence butterfly, etc.); long used by farmers and gardeners for any insect parasite. The O.E. plural in -n (cf. oxen) gradually normalized 13c.-15c. to -s. Slang adj. meaning “clever, alert, wide awake” first recorded 18c., perhaps from the notion of the insect being hard to catch (other theories, however, trace it to fledge or flash); 1990s use may be a revival or a reinvention. Fly on the wall “unseen observer” first recorded 1949. An O.E. word for “curtain” was fleonet “fly-net.” Fly-swatter first attested 1917. Fly-fishing is from 1650s.
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