When a a dude and chick have anal sex and when the guy pulls out, poop is on his dick and he slaps the girl on the cheek with his dick removing all of the poop on his dick and leaving it on her face in the shape of a mushroom.
Jamie: "Dude, David's mushroom stamps hurt!! He hits you so hard with his cock!"
the mark left when one hits their partner in the face with their dick after thuroughly buttsexing them and having a poop covered dipstick, leaving a mushroom shaped mark on their head from the remaining fecal matter.
the AQ krew gangbanged wade and they all mushroom stamped him afterwards