Green’s Dictionary of Slang

piggy bank n.

masturbation.

1968		G. Slatter Pagan Game (1969) 157: The married woman he slept with and when her youngest son climbed into bed with them he, with considerable presence of mind, gave him a piggy back ride which the Padre said was one of the greatest atrocity stories to come out of the war.
1992		R. Puxley Cockney Rabbit.