Specifically, a fly-by-night is someone who departs or flees at night in order to avoid creditors, law enforcement, and so on. A fly-by-night person operates in a dishonest fashion and is not reliable or responsible. A fly-by-night company is a dishonest one that may appear and disappear rapidly.
According to Grose’s Dictionary of the Vulgar Tongue (1796), “fly-by-night” was originally an ancient term of reproach to an old woman, signifying she is a witch. Later, at the beginning of the 1800s, the term referred to anyone who departed hastily from a recent activity, especially while owing money. The disappearance was usually at night, and the person was usually a swindler, whose activities were fraudulent. The person fleeing might also have been a deadbeat tenant who vacated his lodgings in the middle of the night to avoid the wrath of his landlord or other creditors.
The first fly-by-night operator recorded in English makes his appearance in Thomas Love Peacock’s novel Maid Marion, a parody of the Robin Hood legend in which a character refers to Maid Marion and the outlaw as follows: “Would you have her married to an old fly-by-night that accident made an earl and nature a deer-stealer?”
The Encyclopedia of Word and Phrase Origins by Robert Hendrickson (Facts on File, New York, 1977) says that “fly-by-night” has also been, in British slang, a prostitute and a prostitute’s vagina.
According to the Oxford English Dictionary, “fly-by-night” was also used in England in the 1800s as a term for a type of light, usually two-wheeled, carriage. The “fly” (as they were more commonly known) was originally drawn or pushed by a man, later replaced by a horse. That sounds like the English version of the rickshaw.
However, we’re naturally used to thinking of fly-by-night as meaning dishonest, and the phrase is still often used.
I hadn’t thought of fly-by-night as applying to witches, but of course it’s a perfect description. For bats, too, and owls, and any other spooky creatures flying around on Hallowe’en night. Boo!