In military terms, most especially the Navy, merkin is pubic hair left behind on either a toilet seat, or the rim of a men's urinal. More closely associated to women's restrooms than to men's, but applies to both.
A source of great consternation for those assigned the task of 'cleaning the head(restroom)'.
Jay - "Dude, check out all this fucking merkin on the shitter seat!"
Bob - "Thats foul bro! I aint cleaning that up... its your turn!"