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1.
The unfortunate growth of a sex organ originating from inside the rectum and protruding through the anus.
Eww, dude, you have a fuckin' buttpenis. That's nasty.
by Tommy Wommy Womsters May 15, 2003
 
2.
When one's distended asshole gets so blown out, it forms into the shape of a penis. Of course after a while, it begins to scab over and becomes hard which makes it very difficult and painful to wear lower garments of any sort. This will of course have to be removed by surgery or "other means."

This condition is mostly found with homosexual males or whorish women.
Dude, your butt penis is kind of stickin' out there...
by Jonsey Heiman March 25, 2010
 
3.
A penis in which one would assume has recently been inside of someone's butt, judging by sight, smell, or taste.
"Whoa man, I think Jimmy has some serious butt-penis going on!" or "Whoa Jimmy, your dick tastes like some serious butt-penis!"
by Johnny Fitzmacaroni II September 24, 2008