Fly-by-night was originally a noun referring to one who goes out at night,1 usually for some wicked or mischievous purpose. It later gained a slang sense, referring to someone who gets out of a bill or a debt by fleeing in the middle of the night. From this derives the modern sense: today, fly-by-night is usually an adjective describing a fraudulent or dishonest business or money-making scheme. Fly-by-night operations tend to sell shoddy goods or to promise more than they can deliver, and quickly fold or disappear without providing refunds or returns on investment.
From this older sense descends a more general, less negative one: fleeting or insubstantial. For example, we might describe a pop artist with a single catchy song as a fly-by-night artist, or a briefly popular fashion fad as a fly-by-night trend.
The phrase is usually hyphenated, except, obviously, where it’s a verb phrase referring to things that actually fly at night (e.g., “the bats fly by night and sleep by day”). As it’s a well-established idiom, originating in the late 18th century and common in its modern sense since the late 19th, there’s no need to put it in quotation marks in normal use.
When banks close branches in low-income communities, payday lenders, check-cashers, pawnshops and fly-by-night outfits swoop in. [Wall Street Journal]
But this is not some fly-by-night paint job that will wash off after the next rain; it’s 2,000 red, white, and blue shingles, perfectly aligned. [Indiana Curiosities, Dick Wolfsie]
Negligent parents, unscrupulous and inexperienced fly-by-night surgeons, and ignorant and careless attendants have resulted in young initiates suffering from dehydration, septicaemia and gangrene. [Liberator]
While Sharon and Tracey are still living together in a nouveau-riche fly-by-night existence, much has changed. [Herald Scotland]
This should put us immediately out of the realm of the fly-by-night literary magazines that blossom and die with mushroom rapidity here in Paris. [quoted in George, Being George]
The findings included … an $18.8 million no-bid construction contract with a politically connected, fly-by-night contractor who would later default on several performance bonds. [Watchdog]