When you are having sex (preferabbly unprotected) with a girl (must be vaginal) and you cum, you pull out quickly and reach in her pussy. When you grab some of her secretions and also your own, you smear it all over her face and go "Swamp thing!" This is a variation on "WOLFMAN!"
Dude, last night, I totally swamp thing'd this girl. She was so pissed, but I just laughed because she smelled like shit.