Why do they call it a poop deck?

It comes from a French word ‘Poupe’ which is the aftermost part of the ship.

@Odysseus Thank you. :)

Ha, I just realised why turds are called poops,they come from the aftermost part of the body, lol. I just thought it was a nonsensical baby word.

poop, n.1
Forms: 5 pouppe, pope, poppe, 5–7 powpe, 6 pupe, pewpe, 6–7 poup(e, poope, puppe, pup, (7 pub), 7– poop.

[ME. a. OF. pupe, pope (c 1400 in Godef. Compl.), F. poupe = It. poppa, Prov., Sp., Pg. popa:—late L. *puppa for L. puppis poop, stern.]

1. a.1.a The aftermost part of a ship; the stern; also, the aftermost and highest deck, often forming the roof of the cabin built in the stern.

†b.1.b at poop, in (the, one’s) poop, of the wind: Astern. Hence fig. Obs.

c.1.c transf. A cabin built on the after part of the quarter-deck; a round-house. rare.

†2.2 transf. The dickey or seat at the back of a coach; the hinder part of a man or animal, the posteriors, rump. colloq. or vulgar. Obs.

3.3 attrib. and Comb., as poop-cabin, poop-deck, poop-end, poop-ladder, poop-rail, poop-staff; poop-break, the front of the poop of a ship; poop-lantern, -light, a lantern or light carried at the stern to serve as a signal at night; poop-ornament Naut. slang, a ship’s apprentice; poop-royal, the deck forming the roof of the poop-cabin; a top-gallant-poop.

My father, who was serving in the Navy always said, because you were so pooped from all the work. (he was kidding) @Odysseus is right :)

It actually goes back much further than the french “poupe”. The French got it from the spanish “popa”, and that derived from the latin “puppis”, which you can see in use here