Apparently there is no connection at all between the word poop meaning "excrement" and the word poop meaning "topmost deck of the back of a ship". The first definition emerged in the mid-eighteenth century as a children's euphemism. This may be related to an earlier verb meaning "blast", poupen, but it ultimately is thought to be of imitative origin. The second definition of the word emerged at the turn of the fifteenth century and comes from poupe, the French word for "stern of a ship". That, through Old French pope and Italian poppa, traces to the Latin word puppis, still meaning "stern", and the rest of the etymology is unknown. Other definitions of poop include "stupid person" (probably short for nincompoop), a verb meaning "tired" (emerged in the 1930s due to an comparison between exhaustion and defecation) and a kind of sheet used in the US Army (this was somehow associated with the "excrement" definition).
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.