Wanktoss is a derivative of 'Wank' (derived from wanker) and 'toss' (derived from tosser). Both are verbs that mean pretty much the same thing: 'to masturbate'. However, when these two words become Wanktoss it can be used in noun form as an insult or as an exclaimation of pain or annoyance.
Steve: Hey, did you see Hank's hair today?
James: Yeah that guy is such a Wanktoss!