shitstruck

To have a rapid urge to shit when you least expect it.

Sudden urge to shit that makes you squeeze your buttcheeks together tightly to avoid shitting your pants. Usually accompanied by a rapid widening of the eye lids.

Biff: "What's wrong with you, Marty?"
Marty: "I just got shitstruck!"
Biff: "That sucks bro, we are miles from a bathroom."