Mushroom Bump 

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: ...

Mushroom Bump by Teamosabe May 23, 2010

Mushroom bump 

The after effect of slapping a chick in the face with your tallywacker leaving a mushroom shaped bruise

wanna mushroom bump bitch?

Mushroom bump by The warhammer February 7, 2009