Green’s Dictionary of Slang

earner n.

also earn

1. (orig. UK Und.) any job or plan that pays well, almost invariably criminal; often as nice little earner.

[Aus] ‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxiv 4/4: earn: Any money earned by using the wits.
[UK]G.F. Newman You Flash Bastard 106: He’d have thought a couple of poofs would have been able to come up with something more gentle for their earner.
[UK]A. Payne ‘All Mod Cons’ Minder [TV script] 17: Thing is we could be onto a very nice little earner here.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 13: ‘I’m a shitkicker meself, what do youse do for an earn?’ ‘We are students of literature,’ the Charlie volunteered.
[Aus]M.B. ‘Chopper’ Read Chopper From The Inside 117: I made quite a nice little earn over the years by sending so-called hit men to see my enemies with offers to kill me.
[UK]Guardian Sport 2 Oct. 16: The vital little earners that make a pro’s life that little bit easier.
[Aus](con. 1945–6) P. Doyle Devil’s Jump (2008) 143: The boss asked me to [...] put a bit of a scare into you. There was to be an earn in it for me.
[Aus]M.B. ‘Chopper’ Read Chopper 4 175: It would be a fun night out an a good little earner.
[UK]A. Wheatle Dirty South 157: If weed was legalised there’ll be no more shottas and I’d be out of a sweet little earner.
[Aus]T. Peacock More You Bet 10: They would ‘emu’ the grounds [...] and my father assured me that there was a handy three-way earn there every week.
[Aus]D. Whish-Wilson Old Scores [ebook] He’d keep something as insurance, and as a potential doublecross earner.

2. (UK police) a bribe, esp. one paid as regularly as more legitimate wages.

[UK]G.F. Newman Sir, You Bastard 7: His earner had come on top.
[Aus]Bulletin (Sydney) 26 Apr. 44: No monkey, they say, and no way I get bail. So I told em that if I weren’t gonna get bail there’d be no earn for them out of it.
[UK]N. Barlay Curvy Lovebox 52: They got their earners.
[UK]N. ‘Razor’ Smith Raiders 52: Bent Old Bill [...] had to be bunged an earner.

In phrases

on the earn (adj.)

intending to make money by illicit means.

[US]C.S. Montanye ‘Publicity for the Corpse’ in Thrilling Detective Dec. [Internet] They hadn’t gone to the airport as part of any welcoming committee. [...] They were definitely on the earn.