1
general::
1. Tire out, exhaust, as in I ran about ten miles, but then I was too pooped out to go on. 2. Quit, decide not to participate, especially at the last minute. For example, We had about twenty signed up for the seminar, but then half of them pooped out. [Slang; late 1800s]
American Heritage Idioms
2
general::
Phrase(s): poop out
Inf. to quit; to wear out and stop. • He pooped out after about an hour of hard work. • I think I’m going to poop out pretty soon.
McGrawhill's American Idioms And Phrasal Verbs