fly, n.² — Green’s Dictionary of Slang

fly n.²

[SE fly, i.e. one is ‘flying a kite’]

1. a trick, a dodge.

1891

A. Day Mysterious Beggar 271: I put it to him steep; and, my boy, [...] he riz to the fly!

2. (UK und.) constr. with the, any crime involving skill rather than brute force.

1882

‘Some Varieties of Thieves’ in Star (London) 23 Feb. 4/2: The wide doman known among light-fingered gentry as ‘the fly’.

3. (N.Z. prison) one who borrows money or other goods without repaying.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 71/2: fly n. = seagull.

on the fly

1. begging by following passers-by and asking for cash, rather than standing in one place.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 51/2: The ‘first move’ in his mendicant career was taking them on the fly; which means meeting the gentry on their walks, and beseeching or at times menacing them till something is given.
1881		Dundee Courier (Scot.) 25 Feb. 6/5: Brighton is a first-rate place for ‘copping them on the fly’ (begging from people in the streets).

2. getting one’s living by theft, prostitution or some other form of crime.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1895		C. Crowe Aus. Sl. Dict. 54: On the Fly, getting one’s living by thieving.
1931		D. Runyon ‘Earthquake’ in Runyon on Broadway (1954) 146: Sam is on the fly at the time.