Why is poop called poop

First you start with the Poop Deck

The poop is the stern of a ship and the poop deck is the raised deck in the aft of old sailing ships. The word comes from the Latin puppis, meaning stern, via French.

English use of poop dates to the late 15th century. From William Caxton’s 1489 translation of The Book of Fayttes of Armes and of Chyualrye:

The pouppe whiche is the hindermost partye of the shippe.

The term poop deck is somewhat newer, dating to William Sutherland’s 1717 Britain’s Glory: or, Shipbuilding Unvail’d:

All the other Quick work, or Weather work, to have of white, and of black; that is, the Forecastle Deck, and Quarter Deck, Poop Deck, all Plansheers, and about the Timber-heads.

The idea that the poop deck is so named because that is where the heads were located is wrong, both nautically and linguistically. The head, or bathroom, on a ship is so named because the traditional location for them is in the bow of the ship, not the stern. With the wind at one’s back on a sailing ship, locating the toilets in the front would carry the waste forward, away from the ship. Locating them in the stern could produce unfortunate results.

The sense of poop meaning excrement has its origins in nursery slang and is probably echoic in origin, although there are cognates in Dutch and German so it is possible that it is borrowed from those languages. The verb to poop, meaning to make a sound like a horn, can be found in Middle English. From Chaucer’s c.1390 Nun’s Priest’s Tale:

Of bras they broghten bemes, and of box, Of horn, of boon, in whiche they blewe and powped.
(Of brass they brought trumpets, and of boxwood, Of horn, of bone, in which they blew and pooped.)

The verb, meaning to break wind, dates to the late 17th century. From the 1689 Gazophylacium Anglicanum:

To poop, from the Belg. Poepen, to fart softly: both from the sound.

The sense meaning to defecate is even newer, late 19th century. From F.W.P. Jago’s Ancient Language and Dialect of Cornwall:

Poop, or Poopy, to go to stool. (Said by children.)

(Source: Oxford English Dictionary, 3rd Edition)