When your nipples can be seen poking through clothing, often because of cold weather, sexual arousal, or particular types of clothing. Many women wearing bathing suits and sports bras tend to have a problem with this because of thin coverage, feeling uncomfortable at the prominent exposure of their body.
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.