A gamsin is any male body part or appendage. It makes reference - in particular - to the nut sack, gooch, grundel, taint and wang. However, gamsin is a dynamic word. It can be used in any situation to express a certain frustration with one's environment.
Conventional usage: "That girl was fucking crazy, first she slobbered all over my gamsin, then she turned me over and sucked my gamsin!"