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1.
When the tip of the penis is so large and the shaft is so thin, it looks like a mushroom cap.
Condoms feel weird on me because of my large mushroom cap.
by Uncle J J May 23, 2005
 
2.
When the head of the penis is a lot larger the the shaft. Resulting in what looks like the top of a mushroom.
Charlie soon realized that condoms felt odd on his mushroom cap.
by Uncle J J May 25, 2005
 
3.
When two people are having anal sex and the person pitching pulls his penis out of the other one's rectum to find a large glob of santorum hanging from the end of his rod.
I probably should have expected it because we'd gone out drinking all night then eaten that greasy midnight pizza over at Cosmo's, but when we got back to the apartment she let me do anal on her and when I pulled out of her she left a big mushroom-cap hanging off the end of my rod.
by Sock Sniffing Pervert July 03, 2012
 
4.
Noun-The bruise left after a man takes his penis and slaps his partner across the head.

Verb- The act of giving someone a mushroom cap.
Noun- Last night I left a huge mushroom cap on my girl's face.
Verb- I am going to mushroom cap you with my dick!
by Global Aneurysm January 17, 2012
 
5.
an asshole, dickhead
dude don't be such a mushroom cap
by exytsygns December 09, 2007