Word Origin & History
fly (n.)
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. 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. Flypaper attested from 1851, though the item itself is said to have become commonly available in London in 1848.
[flahy-er] 
ər
