Green’s Dictionary of Slang

earner n.

also earn
[1960s+]

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

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

In phrases

on the earn (adj.)

[1940s] intending to make money by illicit means.