The description, fly-by-night, is rarely, if ever, used in a positive sense. It conveys the sense of someone who performs a shoddy job or service, takes the money and disappears. There is an element of untrustworthiness or unreliability about them, especially when the term is used in conjunction with business matters.
The original usage of the word was to denote someone who actually did, or at least according to folk tradition, fly during the night, a witch and by extension, a pejorative term for an old woman. As always, the inestimable Francis Grose in his A Classical Dictionary of the Vulgar Tongue, published in 1788 provides us with a fulsome definition; “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”. Quite how ancient he does not hazard to guess, but as the identification of witches was particularly commonplace in the 17th and 18th centuries perhaps it dates to around that time.
Perhaps inevitably, the term was extended to include other types of women of ill-repute, in particular sex workers and, by extension, that part of their anatomy they traded. These definitions were provided by John Farmer and William Henley in their Slang and its Analogues Past and Present in 1893. They add other usages of the term including “a noctambulist for business or for pleasure; ie a burglar or a common spreester”. Would that noctambulist and spreester return to our daily vocabulary.
The idea that a fly-by-night was someone who was a ne’er do well who is quick to disappear appears to have been in circulation during the 19th century. It may have originated in sporting circles, appearing in John Bee’s Sportsman’s Slang, published in 1825. There Bee, the pseudonym of John Badcock, defines fly-by-night as “run-aways who leave empty houses”. It is tempting to speculate that they were forced into this rather desperate act on account of debts incurred with bookmakers. Farmer and Henley also include this definition – “a defaulting debtor; one who shoots the moon. Also applied to the act” – and, helpfully, provides a gloss on to shoot the moon; “to shoot (or bolt or shove) the moon; to remove furniture by night to prevent seizure for rent”. In other words, doing what we would call a moonlight flit.
There was, though, a third strand of meaning to the term, a carriage. The Morning Post in introducing the term to its readership on April 9, 1818, felt it necessary to define it and also indicate the part of the country in which it was used; “a species of carriage, which, in Gloucestershire, goes by the name of Fly by Night”. What we know as a fly was a light horse-drawn carriage used for public hire and was certainly on the streets of Brighton by 1816. Perhaps the good folk of Gloucestershire were more au fait with the latest modes of transport than others.
The Oxford English Dictionary in 1897 went on to explain the development of this vehicular term; “the name was gradually extended to any one-horse covered carriage, as a cab or hansom, let out on hire”. It was abbreviated in this context to the better-known fly which the OED noted “is generally applied to a vehicle hired from a livery-stable, and not plying for hire”.
Modern usage is restricted to the idea of fleeing the scene, like the defaulting tenant.