When you're at a party and a female is rubbing on you in a passionate way. After a while, you sit down with her and some buddies and she's sitting next to you giving you a hard on. You get up to get some punch, but you have to squeeze through some chairs with some people sitting on them. You squeeze through accidentally slapping someone in the face with your boner and run away fearing to look back.
See also johnson slap
Spike: Yo man!
Bill: What fool?
Spike: Hahaha I just accidentally gave that guy the unruh!