When you've held in a shit so long that you have to run to the bathroom and barely make it on the toilet.
May or may not involve bad cases of diarrhea.
"OMGG!! Get out da way!! I'M SHIT RUNIN'!!!!"
"Dude, my mom got mad at me when I shit ran to the bathroom while we were having 'serious discussion'."
"Oh damn, I think I'm gonna have to shit run soon!"