A case where, due to excessive exercise, a man's penis is smaller than normal. Unlike athlete's foot, athlete's penis is not caused by fungus infections. It happens because all of the blood that would usually keep a man's penis his normal size is being used in his muscles. This is not contagious, but very demoralizing to said man.
Person A: "Ah man, it sucks Harrison got pantsed in from of Allie!"
Person B: "Yeah that was the worst case of Athlete's Penis I've ever seen! I don't know if he'll ever be able to live it down..."