a level of inebriation that extends well beyond that described as "shitfaced," such that bowel control is lost. Typically achieved by drinking on an empty stomach. The exact Blood Alcohol Content required to reach "shitpantsed" drunk is different for each set of bowels.
Cole didn't eat before going to open bar, and wouldn't you know he ended up shitpantsed in the bathtub.
Carl wanted to show everyone he could loosen up, but he'll never live down getting shitpantsed in front of everyone at the holiday party.