When a man has any questionable residue on his penis and, using the head, slaps or "stamps" any region of his partners face... leaving a shroom imprint with the residue. Also applies when a man gains enough force while slapping a person's face with his member, to leave a red mushroom imprint.
Eddie was bored and thought it would be funny to shroom stamp his best friend's forehead... it wasn't.
When a man has been with a woman and his member is in any way tainted by any type of smearable substances and uses the head of his member to apply a shroom stamp to her face. These stamps may be applied gently and lovingly or popped on quickly.
your forehead was the obvious spot for a shroom stamp!