Where a person goes and jacks off in the bathroom of a High school, palms his semen, walks into the cafeteria, and exacts revenge on a person that mushroom stamped the person who is givnig the semen slap. Then slaps the person, with the hand full of semen.
Greg walked into the cafeteria and semen slapped the shit out of him.
When having sex afterwards if you finish on your own you jizz in your hand and slap her in the face with the semen in the hand.
Last night my boyfriend gave me a semen slap.