Online Etymology Dictionary

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. 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.
fly (v.1): "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.
fly (v.2): "run away," O.E. fleon (see flee). Fleogan and fleon were often confused in O.E., too. Mod.Eng. distinguishes in preterite: flew/fled.
fly-by-night: 1796, slang, said to be an old term of reproach to a woman signifying that she was a witch; extended 1823 to "anyone who departs hastily from a recent activity," especially while owing money. The two senses involve the two verbs fly.
flyer: also flier, mid-15c., "something that flies," agent noun of fly (v.1). Meaning "something that goes fast" is from 1795; that of "aviator" is from 1934. Meaning "small handbill or fly-sheet" is from 1889, U.S. slang (originally especially of police bulletins), on notion of "made to be scattered broadcast." Related: Fliers; flyers.
flying: O.E. fleogende, prp. of fly (v.1). Flying buttress is from 1660s; flying fish is from 1510s. Flying saucer first attested 1947, though the image of saucers for unidentified flying objects is from at least 1880s. Flying Dutchman ghost ship first recorded c.1830, in Jeffrey, Baron de Reigersfeld's "The Life of a Sea Officer." Flying colors (1706) probably is from the image of a naval vessel with the national flag bravely displayed.
Flynn: surname, from Ir. flann "red."
flypaper: 1851 (though the item itself is said to have become commonly available in London in 1848), from fly (n.1) + paper.
flywheel: 1784, from fly (n.) "speed-regulating device" (from fly (v.1)) + wheel.
briefly: c.1300, from brief (adj.). As an introduction to a statement, "in short," recorded from 1510s.
butterfly: O.E. buttorfleoge, perhaps based on the old notion that the insects (or witches disguised as butterflies) consume butter or milk that is left uncovered. Or, less creatively, simply because the pale yellow color of many species' wings suggests the color of butter. Another theory connects it to the color of the insect's excrement, based on Du. cognate boterschijte. A fascinating overview of words for "butterfly" in various languages can be found here. The swimming stroke so called from 1936. Butterflies "light stomach spasms caused by anxiety" is from 1908.
The butterfly effect is a deceptively simple insight extracted from a complex modern field. As a low-profile assistant professor in MIT's department of meteorology in 1961, [Edward] Lorenz created an early computer program to simulate weather. One day he changed one of a dozen numbers representing atmospheric conditions, from .506127 to .506. That tiny alteration utterly transformed his long-term forecast, a point Lorenz amplified in his 1972 paper, "Predictability: Does the Flap of a Butterfly's Wings in Brazil Set Off a Tornado in Texas?" [Peter Dizikes, "The Meaning of the Butterfly," The Boston Globe, June 8, 2008]
chiefly: "pre-eminently," mid-14c., from chief (q.v.). Meaning "pertaining to a chief" is from 1870.
dragonfly: 1620s, from dragon + fly (n.).
firefly: 1650s, from fire + fly.
gadfly (n.): 1620s, "fly which bites cattle," probably from gad "goad, metal rod" (early 13c.), here in the sense of "stinger," from O.N. gaddr "spike, nail," from P.Gmc. *gadaz "pointed stick;" but sense is entangled with gad (v.) and an early meaning of gadfly was also "someone who likes to go about, often stopping here and there." Sense of "one who irritates another" is from 1640s (equivalent of L. oestrus).
in like Flynn: 1940s slang, said to have originated in the U.S. military, perhaps from alleged sexual exploits of Hollywood actor Errol Flynn.
superfly: "excellent, superior," 1971, originally U.S. black slang, from super- + slang sense of fly (see fly (v.)).
tsetse fly: 1849, probably via S.African Du., from a Bantu language (cf. Setswana tsetse, Luyia tsiisi "flies").