A mushroom stamp - When you whack someone with your junk and make a giant red m

When you whack someone with your junk and make a giant red mark on their face like a stamp.

My uncle did this to my cousin and it looked like a facepalm.

At the party, a guy hit another guy with his cock and it looked like a slap.

My brother did this to his friend and he had a red face like a tomato.