The noun fly-by-night, or fly-by-nighter, denotes an unreliable or untrustworthy person. As an adjective, fly-by-night means unreliable or untrustworthy, especially in business or financial matters.

However, the term seems to go back to the idea of witches flying on their broomsticks by night. At least, its first recorded instance is as a term of contempt for an old woman compared to a witch—from A Classical Dictionary of the Vulgar Tongue (1788 edition), by the English antiquary and lexicographer Francis Grose (1731-91):

Fly-by-night. You old fly-by-night; an ancient term of reproach to an old woman, signifying that she was a witch, and alluding to the nocturnal excursions attributed to witches, who were supposed to fly abroad to their meetings, mounted on brooms.

In Slang and its Analogues Past and Present (1893), John Stephen Farmer (1854-1916) and William Ernest Henley (1849-1903) added that fly-by-night meant “a prostitute” and “the female pudendum”; they also gave the following definition:

A noctambulist for business or for pleasure: i.e. a burglar or a common spreester*.

(* spreester: one who goes on a spree, in the sense of a boisterous frolic)

But the current sense of fly-by-night seems to have its origin in another usage of the word. The same dictionary has this definition:

A defaulting debtor; one who shoots the moon. Also applied to the act.

The phrase to shoot the moon was thus explained by Farmer and Henley:

To shoot (or bolt or shove) the moon: to remove furniture by night to prevent seizure for rent.

In Sportsman’s Slang; A New Dictionary of Terms used in the Affairs of the Turf, the Ring, the Chase, and the Cock-Pit (1825), John Badcock, writing under the pseudonym of Jon Bee, had already given the following definitions:

– Fly-by-night—run-aways who leave empty houses. Look at ‘Moon-light.’
– ‘Moonlight—wanderers;’ or ‘fly-by-night’ persons, who cheat their landlords and run away by night; when ’tis illegal to detain the goods.

But there was yet another usage of fly-by-night. The Morning Post (London) of 9^th April 1818 mentioned

a species of carriage, which, in Gloucestershire, goes by the name of a “Fly by Night.”

This usage of fly-by-night, or simply fly, was thus defined by the New English Dictionary (i.e. the Oxford English Dictionary – 1897 edition):

Fly. The name of a light vehicle, introduced at Brighton in 1816, and originally drawn or pushed by men; but a horse being soon employed, the name was gradually extended to any one-horse covered carriage, as a cab or hansom, let out on hire. Perhaps short for fly-by-night.
Local usage of the word varies; in some places fly is confined to a ‘four-wheeler’; but it is generally applied to a vehicle hired from a livery-stable, and not plying for hire.