Origin and history of fly

[winged insect] Middle English flie (2), from Old English fleoge, fleogan "a fly, winged insect," from Proto-Germanic *fleugon "the flying (insect)" (compare Old English fleogende "flying"). According to Watkins this is from PIE root *pleu- "to flow," which is also the reconstructed source of fly (v.1). The plural flien (as in oxen, etc.) gradually was normalized 13c.-15c. to -s. 

Originally and in popular language a flying insect of any common kind (moths, gnats, bees, beetles, locusts, hence butterfly, etc.) and long used by farmers and gardeners for any insect parasite. Especially of the common house-fly (Latin musca). In modern entomology, a two-winged insect of the order Diptera especially of the family Muscidae.

Flies figuratively for "large numbers" of anything is from 1590s. Fly in the ointment "small or trifling matter which spoils enjoyment" is from Eccles. x:1. Fly on the wall "unseen observer" is recorded by 1881. No flies on _____ "no lack of activity or alertness on the part of," is attested by 1881, said to be a reference to active cattle.

The meaning "fish-hook dressed to resemble an insect" is attested from 1580s; fly-fishing is from 1650s; fly-rod, used by anglers in fly-fishing, is from 1680s.

The fly agaric mushroom (1788) is so called because it was used in fly powders and other poisons for flies.

Germanic cognates include Old Saxon fleiga, Old Norse fluga, Middle Dutch vlieghe, Dutch vlieg, Old High German flioga, German Fliege "fly." 

[pass or rise swiftly through air; move through the air with wings] Middle English flien, from Old English fleogan, fliogan "move through the air by the aid of wings; rise into or move over the air by the force of wind or other impulse," which is reconstructed to be from Proto-Germanic *fleugan "to fly," reconstructed in Watkins to be from PIE *pleuk-, extended form of root *pleu- "to flow."

Not cognate with flee (v.), but inextricably confused with it since Old English times due to their similarity, which once was greater than now.

OE flēogan 'fly' and flēon 'flee' differed only in the stem of the present. In some dialects of ME the two verbs seem to have coalesced completely. [Middle English Compendium]

For the senses of "depart suddenly or swiftly, take flight" (c. 1200); "flee from, shun" (trans.), etc., see fly (v.2).

The sense of "go at full speed, move or pass with swiftness or alacrity" is attested from c. 1200. In reference to time from 1590s.

Of persons or animals, "spring, rush, move with a start," by 1580s.

It is attested by c. 1300 in the sense of "part suddenly or with violence, burst into fragments" (as in fly to pieces, itself from late 15c.), probably from the notion of the fragments scattering through the air.

In reference to flags, ship's colors, etc., "float loosely, flutter as in the wind," by 1650s (thus some of the senses under fly (n.2)).

The transitive sense "cause to rise and move or float in air" is by 1739 (fly a kite); that of "convey through the air" ("Fly Me to the Moon") is by 1864.

Specifically in reference to travel by mechanical aircraft by 1884. In reference to any sort of aircraft, "make a flight," by 1848.

To fly at "attack, spring or rush at with hostile intentions" (1580s, earlier fly upon, 1540s) is perhaps from hawking. To let fly, let fly at (someone) "make an attack or assault; shoot (an arrow), hurl (a stone)" is attested by c.1200.

Figurative fly too high "be too ambitious" is by early 15c.

Germanic cognates include Old Saxon fliogan, Old Frisian fliaga, Middle Dutch vlieghen, Dutch vliegen, Old High German fliogan, German fliegen, Old Norse flügja. 

Also see flying.

Old English fleogan was a class II strong verb; past tense fleag, past participle flogen, the modern flew and flown. Flied is used in specialized senses, as in baseball ("hit a fly ball") or "to ride the flys" an old type of light hackney carriage (1836).

"run away," Old English fleon, flion "fly from, avoid, escape;" in this use the modern word is essentially a variant spelling of flee (q.v.).

In Old English, this verb and fleogan "soar through the air with wings" (modern fly (v.1), q.v.) differed only in their present tense forms and often were confused.

They have been distinguished from one another since 14c. in the past tense: flew (occasionally flied) for fly (v.1), fled for fly (v.2).

To fly the coop "escape, depart at once" is U.S. colloquial attested by 1887, a reference to caged birds and etymologically perhaps involving both verbs.

slang, "clever, alert, wide awake," by 1811, perhaps from fly (n.1) on the notion of the insect being hard to catch. Other theories, however, trace it to fledge or flash. Slang use in 1990s might be a revival or a reinvention.

"a flight, act of flying or passing through the air," mid-15c., flie, from fly (v.1) or Old English flyge, which corresponds to Old High German flug (see flight (n.1)).

From the verb in the sense of "float loosely, flap as a wing does" comes the noun sense of "something attached by one edge" (compare flap). Hence "flap or door on a tent" (1810), extended to "strip of material sewn into a garment as a covering for buttons" or some other purpose (1844).

Theatrical flies for the space over the proscenium extending over the stage is by 1805.

The notion of "something having a rapid or flying motion" led to widespread technical senses in mechanics, printing, weaving, baseball, etc. Compare flywheel. Colloquial give it a fly "make an effort" is by 1917 in Australian English.

From the notion of "during flight, in the air, before reaching ground" comes the baseball fly ball, attested by 1866.

When the catcher sees several fielders running to catch a ball, he should name the one he thinks surest to take it, when the others should not strive to catch the ball on the fly, but only, in case of its being missed, take it on the bound. ["The American Boys Book of Sports and Games," New York, 1864]

To do something on the fly, implying some degree of improvised action, is by 1856, apparently from baseball but it also might refer to shooting game-birds in flight.