When you have an erection for so long it begins to hurt.
The only known cure for this is sex, masturbation will postpone the effects but not stop it all together.
Most commonally affects young teenagers with glasses, inhalers and a chess award.
Dude 1: Dude can I fuck your sister, I'm getting serios penis ache here
Dude 2: Bugger off I'm curing mine first