Green’s Dictionary of Slang

1. (US) in a hurry, on the move, spontaneously.

1842		Edinburgh Rev. July 484: Cadgers on the fly are those who beg as they pass along the tober (road). Cadging on the fly is a profitable occupation in the vicinity of bathing-places and large towns.
1868		‘Six Years in the Prisons of England’ in Temple Bar Mag. Nov. 537: Picking pockets [...] is more difficult on the ‘fly’.
1874		‘English Sl.’ in Eve. Telegram (N.Y.) 9 Dec. 1/5: Let us present a few specimens:– [...] ‘On the fly.’.
c.1880		‘US Army Sl. 1870s–1880s’ [compiled by R. Bunting, San Diego CA, 2001] On the fly In a hurry; in motion.
1935		J. Conroy World to Win 62: I nails a grab iron on the fly.
1940		B. Appel People Talk (1972) 272: You eat your dinner on the fly so you can finish drillin’ by four o’clock.
1954		Kerouac letter 2 Jan. in Charters I (1995) 427: And my God if Burroughs goes there I’m sure to come on the fly.
1969		M. Puzo Godfather 145: I’ll spot you and put on my headlights and catch you on the fly.
1977		L. Heinemann Close Quarters (1987) 47: I snatch up the strap of the pig on the fly.
1989		J. Morton Lowspeak.
1993		Eble Campus Sl. Apr. 4: on the fly – hurriedly. ‘I ate a burger on the fly.’.
1999		Guardian Rev. 15 Oct. 21: His on-the-fly imagination.
2012		L. Berney Whiplash River [ebook] Gina’s abiliy to lie on the fly was scary.
2013		T. Robinson Hard Bounce [ebook] ‘We’ll do this on the fly. let’s see where she goes first’.

2. (US tramp) while moving, usu. describing the boarding of a (freight) train; often as catch it on the fly.

1907		J. London Road 159: Wait till you hit the Pennsylvania, four tracks, no water tanks, take water on the fly, that’s goin’ some.
1917		‘A-No. 1’ From Coast to Coast with Jack London 24: See our train taking water on the fly!
1929		E. Dahlberg Bottom Dogs 178: He caught a streetcar on the fly which happened to be going down to the railroad station.
1930		C.R. Shaw Jack-Roller 128: I waited at the depot and caught the Omaha Limited ‘on the fly’.
c.1935		J. Wright ‘Battler’s Ballad’ in Seal (1999) 97: With your swag held at the ready, your nerves are not so steady / For you know you’ve got to take her on the fly.
1966		B. Jackson Get Your Ass in the Water (1974) 71: I’ve toted a pack down a B and O track / hopped redball freights on the fly.
1990		(con. 1930s–60s) H. Huncke Guilty of Everything (1998) 238: We waited ’til they linked up the various cars and gave the highball, and then we caught it on the fly.

3. cunningly, clandestinely, in secret.

1929		C. McKay Banjo 317: They were all going ‘on the fly’ and none of them was thinking of staying with the boat after the trip, but rather of getting to Cuba, Canada, and the United States.
1930		in E. Cray Erotic Muse (1992) 190: So we pulled out our guns and we got him on the fly, / Crawled in the weeds, and I guess he’s going to die.
1949		N. Algren Man with the Golden Arm 17: He had snatched snipes, on the fly.
1950		‘Josephine Tey’ To Love and Be Wise 154: ‘You got the car out on the fly?’ ‘No,’ Grant said, and asked why she should think that. ‘Oh [...] Thought maybe you were out for the day on your little own’ .
1993		I. Rankin Black Book (2000) 133: He knew they were decorators because they’d asked him if he needed any work doing. ‘On the fly, like. Cheaper that way.’.

4. by mistake.

1975

E. Torres Carlito’s Way 126: I’m here on the fly, caught up in the web.

5. (US black) living in an expensive, fashionable, self-indulgent manner.

1994

G. Smitherman Black Talk.