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.
"to soar through air," O.E. fleogan (class II strong verb; past tense fleag, pp. flogen), from W.Gmc. *fleuganan (cf. O.H.G. fliogan, O.N. flügja, M.Du. vlieghen, Ger. fliegen), from PIE *pleu- "flowing, floating" (cf. Lith. plaukiu "to swim"). Notion of "flapping as a wing does" led to noun sense
of "tent flap" (1810), which yielded (1844) "covering for buttons that close up a garment." Slang phrase fly off the handle "lose one's cool" dates from 1825. On the fly is 1851.
"run away," O.E. fleon (see flee
). Fleogan and fleon were often confused in O.E., too. Mod.Eng. distinguishes in preterite: flew/fled.