V. To slap or whack a person in the face with your penis. Normally leaves a mark, also known as a mushroom tattoo. Sometimes done during a blow job if it is a bad one, as to punish the person for doing a bad job.
EX1: Dude i was walking down the street when some guy yelled "SHROOM WHACK" and then i got slapped with his cock!
EX2:Bob- Dude I shroom whacked her so hard last night, it left a mark.
Jim- big deal, i shroom whack my lady every night.
Also see cock slap