When you slap someone with your soft circumcised penis causing a small red mushroom shaped mark on the skin from the head of the penis.
Dude I mushroom slapped her last night right on the cheek, it left a red mark!
ayon kay Aiden07321 ika-06 ng Hunyo, 2010
1 more definition
To awaken one under the influence of psilocybin from incoherence with a quick slap of the hand to the face
Tony mushroom slapped Albert so as to awaken him from his incomprehensibly negative, drug-induced rant. (But Bert just became further enraged and had to be wheel chaired to bed.)
ayon kay Alphonse Drizzny ika-05 ng Enero, 2009