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2.
if someone happens to be getting fresh with you, mushroom slap them. This means pulling out your jimmy and making solid contact with their facial region.
C-dav definitely mushroom slapped that girl last period because she said "oh my gosh guys guess what happened?"
by drew s April 12, 2004
 
1.
To slap someone across the face with one's penis.
Those dudes are always mushroom slapping each other.
by Nate Fergus February 02, 2003
 
3.
Being slapped with a wang. Usually a hard wang.
"So this chick was blowin' some guy one day, and he pulled out and gave her a mushroom slap!"
by D.Socko May 08, 2004
 
4.
When a guy takes his hard dick, and swings it as hard as he can, hitting a person (hopefully a girl) on her body with his mushroom. If done correctly it will leave a mushroom imprint for several minutes with a lasting red spot afterwards.

(not easy to do)
ex) mushroom slap her in the face and it will leave a mark. very funny. total ownage
by Mushroom slap 696969 May 14, 2009