poop 2 
(pp)
(pp)tr.v. pooped, poop·ing, poops
Slang
Phrasal Verb: Slang
To cause to become fatigued; tire: "Many people stop here, pooped by the short, steep climb" (Sierra Club Guides to the National Parks).
poop out Slang
1. To quit because of exhaustion: poop out of a race.
2. To decide not to participate, especially at the last moment.
[Origin unknown.]