Why is a poop deck called a poop deck?

The term 'poop deck' refers to the partial deck above the ship's main afterdeck at the stern of the ship. In the days before indoor plumbing, a structure was rigged aft of this deck over the sea for the purpose of relieving oneself. When a ship takes a wave over the stern of a ship it is said to be 'pooped.'

https://www.pbo.co.uk/nautical-almanac/glossary-of-nautical-terms/pooped-to-be-8789

I'd like to think that the derivation from ships' lavatories is true, but the Oxford English Dictionary and Chambers derive the word from the Latin 'puppis', meaning the stern of a ship.

Lavatories on ships are known as the 'Heads' , these were traditionally at the front end of the ship.

I once sailed on the New Endeavour fpgjc.
It had a large sized board
with a hole in the centre to sit in,
on either side of the bow
to do the what comes naturally .

Imagine the saving of not having to use toilet paper in the middle of a storm with gigantic waves.
You'd be flushed with pride!
Get a shower at the same time.
Take your soap.
Wash your socks.
Shrink your googlies.

Because there were two holes you could have a chat with your mate as well !
Read the newspaper !
Do the crosswords !
Captain James Cook was a genius!.

'At the inboard side of the catheads are the open lavatories, for the use of 76 of the 94 aboard. These 'seats of ease' are today known as the 'heads' because of the location at the head of the ship. Under the bowsprit of the original Endeavour would have been a half-barrel, with a lid, which was used to collect urine. The seamen would wash their clothes in the urine to kill the lice prior to rinsing them in seawater.'

Poop deck is a term used in a naval architecture to define the roof of the cabin on the back part of the ship. A poop deck is essentially the stern deck of the ship. In a sailing ship, this poop deck is usually elevated from the roof. It is therefore easier for the captain or the sailor to have a view of the crew and have better navigation.

https://en.wikipedia.org/wiki/Poop_deck