Green’s Dictionary of Slang

(US) an excl. of annoyance or disappointment.

1972		A. Theroux Three Wogs 111: The train would be late, now; he felt disappointed that he felt sad, sad that he felt disappointed. [...] Poop, he thought, I should have known this.
1987		S. King Misery (1988) 13: But he said, ‘It changed its mind.’ ‘Oh, poop!’ I said.