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1.
To take a moderate dose of cocaine ("bump") off the head of an erect penis ("mushroom").
Tom: Dude, you see that guys' boner?
PJ: No, Tom give up the dream
Tom: Imma go over to him and ask to give him a mushroom bump
PJ: what's a mushroom bump?
Tom: it's where you take a bump of cocaine off the head of another man's boner!
PJ: ...
by Teamosabe May 23, 2010
 
2.
The after effect of slapping a chick in the face with your tallywacker leaving a mushroom shaped bruise
wanna mushroom bump bitch?
by The warhammer February 06, 2009