I GOTTA TAKE A SHIT
A loud and smelly excuse to leave right then and there. It means you're not coming back for a while, and you're probably going to be extra smelly when you return.
I gotta take a shit. See you when I'm done and probably stinking up the whole house.
I gotta take a shit. Don't even think about touching my pizza.
I gotta take a shit. I might take the whole bathroom with me.