Green’s Dictionary of Slang

1. dubious, untrustworthy, undependable.

1881		Daily L.A. Herald 13 Aug. 2/3: A ‘flybynight’ manager is one without money, and a wealthy one is called a ‘solid fixer‘.
1911		S. Ford Torchy 109: Most of ’em was little, two-room, fly-by-night firms, with a party ’phone [...] and a mail-order list bought off’m patent medicine concerns.
1912		A.G. Field Watch Yourself Go By 401: I’ll never take another chance with a fly-by-night troupe.
1914		J. London letter 25 June in Letters (1966) 425: I have no patience with fly-by-night philosophers such as Bergson.
1927		(con. 1900s) S. Lewis Elmer Gantry 167: This fool idea that a lot of these fly-by-night firms are hollering about now.
1931		T. Thursday ‘Dead Steal’ Detective Fiction Weekly 13 June 🌐 The stranger had studied the six motors on the fly-by-night used car lot.
1947		S. Lewis Kingsblood Royal (2001) 175: A fly-by-night joint like this.
1958		E. Dundy Dud Avocado (1960) 92: Listen you bums, this is no fly-by-night proposition.
1969		N. Spinrad Bug Jack Barron 27: Fly-by-night outfits.
1971		Guardian 12 July 7/3: It is not all heart in the mini-cab world. Far too many are fly-by-night hustlers.
1976		Cab Calloway Of Minnie the Moocher and Me 47: You’re too fly-by-night to be getting married.
1982		J. Sullivan ‘A Touch of Glass’ Only Fools and Horses [TV script] He ain’t one of your fly-by-night merchants.
1986		O. Senior ‘Ballad’ Summer Lightning 124: She say that he wasnt no hothead little fly-be-night bwoy, he was an establish man.
1988		S. Selvon Eldorado West One 69: You shock me Moses! What I have in mind is no fly-by-night affair.
2003		(con. 1932) W. Woodruff Beyond Nab End 36: I’ve landed a job as a weaver in a fly-by-night place in west London.

2. crooked, criminal.

1940

D. Maurer Big Con 21: A host of fly-by-night places came and went.