Nob nipping
When a bloke has half a hard on and sits down to take a piss. He adjusts on the seat to stop getting piss all over the place and his nob slips under the rim of the seat, thus when he sits back down he squashes his nob between the seat and the rim.
Nob nipping - usually happens in the morning after a hard night on the piss, or after a shag or to blokes that are well endowed. May in fact be the only down side to having a huge penis.