What is the etymological derivation of a 'poop deck' on a ship?

Nothing to do with where the bathroom is located. Poop comes from the Latin puppis, meaning "stern." Ships often had a cabin space in the very back of the ship called a poop cabin. The poop cabin extends a few feet above the main deck, and has a flat roof. The flat roof of a poop cabin is an observation platform called the poop deck.