Poop out
Definition 1 of 2
Pro Tip 1/3Informal Usage
Best used with friends in casual conversations, not in formal settings. 
Sorry I can't hang out, I pooped out after the gym.
Pro Tip 2/3Not Literal
Doesn't refer to bodily functions, but to feeling very tired. 
I wanted to finish the book, but I pooped out by chapter three.
Pro Tip 3/3Physical Activities
Often used when speaking about physical activities like sports or exercise. 
She pooped out halfway through the marathon.