It's when you get gangreen on your penis. It's disturbing and probably means you will never get laid again. Also, don't even try to get a boner when you have it. Sadly, chopping it off is the only way to get rid of it. I do not know how you would get gangreenis, except if you were buck naked and running through antarctica.
I hope I never see that guy's gangreenis again! It ranked in the top 5 of most disturbing things I have ever seen!