The absolute utter horror a person experiences when they are at school and for whatever reason they piss their pants. This phenomena usually occurs after a laughing spell or some sort of frightful event that startles the pisser. Usually this person is somewhat of a pussy for the fact that he cannot hold in his pee for the next hall break. The horror piss is followed by a long trip to the bathroom in which the pisser desperately tries to get the extremely embarrassing stain out of his pants but wiping down with paper towels or placing crotch under hand dryer. If somewhat is spotted while horror piss stain is intact, their social life is ruined.
Harry: "Yo man did you see that chick, she musta had a bad horror piss"