(plural flys). A hackney coach, a cab. A contraction of Fly-by-night, as sedan chairs on wheels used to be called in the regency. These “Fly-by-nights,” patronised greatly by George, Prince of Wales, and his boon companions, during their wild night pranks at Brighton, were invented 1809 by John Butcher, a carpenter of Jew Street.
“In the morning we took a fly, an English term for an exceedingly sluggish vehicle, and drove up to the Minister's.” —Hawthorne: Our Old House (Pilgrimage to Old Boston, p.171).
(plural flies). An insect. All flies shall perish except one, and that is the bee-fly. (Koran.)
ACHOR, god of the Cyrenians, to whom, according to Pliny, they offered sacrifice. (APOMYIOS, a surname given by the Cyrenians to Zeus, for delivering Herakles [Hercules] from flies during sacrifice. Sacrifices were yearly offered to Zeus Apomyios. (Greek, apo-myia, from flies.)
BELZEBUB or BEELZEBUTH (Prince of Flies) was one of the principal Syrian gods, to whom sacrifice was offered on all ferialia.
BUCLOPUS, in Roman mythology. (Rhod. xxii. 3.)
MYAGROS (the fly-chaser), one of the deities of the Arcadians and Eleans. (Pliny, x. 28.) (Greek, myia a fly; agra, taken in hunting or chasing.)
“He [the professor] rose to the fly with a charming simplicity.” —Grant Allen: The Mysterious Occurrence in Piccadilly, part ii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894