Green’s Dictionary of Slang

a trick, a scheme.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 272: suit: in general synonymous with game; as, what suit did you give it to ’em upon? in what manner did you rob them, or upon what pretence, &c., did you defraud them? One species of imposition is said to be a prime suit, another a queer suit: a man describing the pretext he used to obtain money from another, would say, I draw’d him of a quid upon the suit of so and so, naming the ground of his application. [...] A person having engaged with another on very advantageous terms to serve or work for him, will declare that he is upon a good suit. To use great submission and respect in asking any favour of another, is called giving it to him upon the humble suit.
1821		Egan Life in London (1869) 352: Indeed, upon this suit some of the best judges in the kingdom have been ‘had.’ Inviting a man to a swell dinner, and making him pay five guineas a mouthful for it afterwards, is no new feature in life in london. It is three playing one; and jerry stood the nonsense in prime style.
1835		Metropolitan Mag. XIV Sept. 333: Our suit [...] got on pretty well — we served it out to three flatty-gories in the first week, clying upwards of a hundred couple of quid.