Dick

A penis, cock, member, cum squirter 3000, cum-gun or shlong. Can also be used to describe an idiot.
Or the more formal method is dickhead. Just call someone idiotic a dickhead and see what happens. One of two things. Either you will get a very good reputation and people fear you, or you will get slamdunked out the window.

Jay: mate that geezer was chatting me up on the bus
Ben: Ryan? That dick