Third person singular of poop; defecates, or becomes exhausted (informal).
The excrement meaning comes from Middle English 'poupen' meaning to make a gulping sound. The exhaustion sense ('pooped out') developed in early 20th century American English, possibly from the nautical term for the stern deck.
The nautical connection to the 'poop deck' (ship's highest rear deck) creates amusing confusion! Sailors standing watch on the poop deck would indeed become exhausted, potentially linking the maritime and fatigue meanings of this versatile word.
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