poop (n.1)

1670s, nicompoop; modern form from 1713. Despite similarity [noted by Johnson] to Latin legal phrase non compos mentis "insane, mentally incompetent" (c. 1600), the connection is denied by the OED's etymologists because the earliest forms lack the second -n-. Weekley thinks first element may be a proper name, and cites Nicodemus, which he says was used in French for "a fool," or Nicholas. Klein says it is probably an invented word. Century Dictionary has no objection to the non compos mentis theory.

"tired," 1931, of unknown origin, perhaps imitative of the sound of heavy breathing from exhaustion (compare poop (n.2)). But poop, poop out were used in 1920s in aviation, of an engine, "to die." Also there is a verb poop, of ships, "to be overwhelmed by a wave from behind," often with catastrophic consequences (see poop (n.1)); hence in figurative nautical use, "to be overcome and defeated" (attested in 1920s).

It is an easy thing to "run"; the difficulty is to know when to stop. There is always the possibility of being "pooped," which simply means being overtaken by a mountain of water and crushed into the depths out of harm's way for good and all. [Ralph Stock, "The Cruise of the Dream Ship," 1921]