Describes the acute onset of the urgent need to shit. In most cases, diarrhea. This is the kind of shit that you simply can't hold it. Your ass cannot relax or there will be a disaster.
person 2: (standing up while driving) "I'm about to seriously shit my pants! I gotta get to a bathroom!!!"
person 1: "yep, sounds like you have a serious case of S.S.S. my friend."