Green’s Dictionary of Slang

1. corruption.

1903

Sun. Times (Perth) 26 July 1/1: [headline] Snide Auctioneers. Of sharks and sharkings in Sydney there seems no end.

2. the practice of a private credit company taking high interest on loans.

1973		G.V. Higgins Digger’s Game (1981) 146: ‘What’s the business?’ ‘Junkets and sharking.’.
1981		S. King Roadwork in Bachman Bks (1995) 365: Dope, girls, gambling [...] sharking.