horsefly n.

"solidungulate perissodactyl mammal of the family Equidæ and genus Equus" [Century Dictionary], Old English hors "horse," from Proto-Germanic *harss- (source also of Old Norse hross, Old Frisian, Old Saxon hors, Middle Dutch ors, Dutch ros, Old High German hros, German Roß "horse"), of unknown origin. By some, connected to PIE root *kers- "to run," source of Latin currere "to run." Boutkan prefers the theory that it is a loan-word from an Iranian language (Sarmatian) also borrowed into Uralic (compare Finnish varsa "foal"),

The usual Indo-European word is represented by Old English eoh, Greek hippos, Latin equus, from PIE root *ekwo-. Another Germanic "horse" word is Old English vicg, from Proto-Germanic *wegja- (source also of Old Frisian wegk-, Old Saxon wigg, Old Norse vigg), which is of uncertain origin. In many other languages, as in English, this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in Indo-European religion. For the Romanic words (French cheval, Spanish caballo) see cavalier (n.); for Dutch paard, German Pferd, see palfrey; for Swedish häst, Danish hest see henchman. As plural Old English had collective singular horse as well as horses, in Middle English also sometimes horsen, but horses has been the usual plural since 17c.

Used at least since late 14c. of various devices or appliances which suggest a horse (as in sawhorse), typically in reference to being "that upon which something is mounted." For sense of "large, coarse," see horseradish. Slang use for "heroin" is attested by 1950. To ride a horse that was foaled of an acorn (1670s) was through early 19c. a way to say "be hanged from the gallows." Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Horse-pistol, "large one-handed pistol used by horseback riders," is by 1704. A dead horse as a figure for something that has ceased to be useful is from 1630s; to flog a dead horse "attempt to revive interest in a worn-out topic" is from 1864.

HORSEGODMOTHER, a large masculine wench; one whom it is difficult to rank among the purest and gentlest portion of the community. [John Trotter Brockett, "A Glossary of North Country Words," 1829]

The term itself is attested from 1560s. The horse's mouth as a source of reliable information is from 1921, perhaps originally of racetrack tips, from the fact that a horse's age can be determined accurately by looking at its teeth. To swap horses while crossing the river (a bad idea) is from the American Civil War and appears to have been originally one of Abe Lincoln's stories. Horse-and-buggy meaning "old-fashioned" is recorded from 1926 slang, originally in reference to a "young lady out of date, with long hair." To hold (one's) horses "restrain one's enthusiasm, be patient" is from 1842, American English; the notion is of keeping a tight grip on the reins.

Old English fleoge "a fly, winged insect," from Proto-Germanic *fleugon "flying insect" (source also of Old Saxon fleiga, Old Norse fluga, Middle Dutch vlieghe, Dutch vlieg, Old High German flioga, German Fliege "fly"); literally "the flying (insect)" (compare Old English fleogende "flying"), from PIE root *pleu- "to flow," which is also the source of fly (v.1).

Originally any winged insect (moths, gnats, beetles, locusts, hence butterfly, etc.) and long used by farmers and gardeners for any insect parasite. Flies figuratively for "large numbers" of anything is from 1590s. Plural flien (as in oxen, etc.) gradually normalized 13c.-15c. to -s. Fly in the ointment is from Eccles. x:1. Fly on the wall "unseen observer" first recorded 1881. No flies on _____ "no lack of activity or alertness on the part of," is attested by 1866. Meaning "fish-hook dressed to resemble an insect" is from 1580s; Fly-fishing is from 1650s. Fly-catcher "bird which eats insects on the wing" is from 1670s. The fly agaric mushroom (1788) so called because it was used as a poison for flies.

The sense of "a flight, flying" is from mid-15c. From the verb and the notion of "flapping as a wing does" comes the noun sense of "tent flap" (1810), which was extended to "strip of material sewn into a garment as a covering for buttons" or some other purpose (1844). Baseball fly ball attested by 1866. To do something on the fly is 1856, apparently from baseball.

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]