The same as a normal pimp-slap, where you assault your target by slapping him/her with the back of your hand, except you run your hand down your own butt crack 1st. This marginalizes and denigrates the offender even further.
I was so upset with this loudmouth drunk at a bar, that I had to shut him up; so I dirty pimp-slapped him into submission.
When you're fucking you're girlfriend, you scream a distraction and make her turn her head. At that very moment you pull your dick out, and slap her across the face when she turns back. Make sure you haven't washed your dick for at least a week.
Dude, did you see that huge red mark on Jessica's face? Yeah, I dirty sausage slapped that bitch.