A bruise, in the shape of a mushroom, obtained by being slapped with a large penis or penis-like object.
"Dude, I can't believe you goin' out with the new girl from accounting!"
"I'm gonna mushroom-bruise her ass, just for you. Just watch -- she won't be sittin' straight for a week."
The act of pulling out your penis while being blown, and systematically slapping your partner's face with it, thus leaving a purplish bruise resembling the tip of you penis (a mushroom).
I gave Carly a purple mushroom bruise when she went down on me last night.