The unpleasant moment that occurs when a mans "scrumptious hard nipples" appear through his tight shirt. This moment can be made much worse when the man bears "man-titties". This appearance will normally cause cringe, disgust, or hilarity to others.
Patrick: Bro, did you see that? That sight will never leave my mind.
Juan: Damn what'd I miss?
Patrick: I just witnessed an unfortunate nip-slip
A sexual partner used as exercise equipment, specifically for aerobic, standing, rear-entry penetration. To qualify, the penetrator must manipulate his partner's nipples.