Why is the poop deck called a poop deck?

Answer 1

It is named after the French word for stern, la poupe. It is the roof of a superstructure cabin at the aft end of the ship.

It comes from the latin word puppis, which means just what it is -- an enclosed deck at the stern of a ship.

or

Pirates used to use it as a bathroom.

Answer 2

It just means the stern of a ship, from the Italian Poppa. But otherwise of uncertain origin.