Green’s Dictionary of Slang

a masturbator; thus a general term of abuse.

1999		K. Sampson Powder 132: ‘We ain’t had a banker like Helmet in years ...’ ‘Merchant Banker, yeah!’.
2000		Guardian G2 20 Jan. 16: There appear to be few instances where a job lends itself to a home-made Cockneyism. ‘Merchants’ (as in ‘merchant bankers’) is the most obvious.
2003		B. Dark Dirty Cockney Rhy. Sl. 80: I had to quit my job because the bosses were all a bunch of merchant bankers. [Ibid.] 97: Swiss banker wanker.