It is a deck at the rear of a ship, that takes its name from the Old French word pupe, meaning stern.
I was watching a movie on the weekend and it was set on a ship (a Stanley Tucci film called The Imposters). In the background, there was a sign with an arrow directing people to the poop deck. It’s a phrase I’ve heard before, but I don’t spend much time on ships (or boats), so I didn’t know what that is so I looked it up.
(also poop deck)
The aftermost and highest deck of a ship, especially in a sailing ship where it typically forms the roof of a cabin in the stern.
‘there on the poop stood Captain Meech’
It turns out that, in this context, poop is derived from the Old French word pupe, meaning stern.
According to Merriam Webster, the phrase poop deck dates back as far as the early 18th century.