There was not a great deal found on this topic, which is weird since the word has so much influence on our culture. However, it looks like etymologist expert Eric Partidge covered this word briefly in this book, Origins: A Short Etymological Dictionary of Modern English. According to Partridge the word poop was formed from the Middle English word poupen or popen. Poupen originally meant “fart”. This word was used for the word fart because apparently that is how farts sounded in the 1800’s.
Our diets must have changed so much since then because I have never heard a fart sound like “poupen”. If we had to name farts today based on sound I might choose “bbbmaaerrrfhhhh….ppzztff!”
However, before reading up on the subject, here at NTG we decided to take our own guess at how the word came about and realized it is very hard to determine where a word comes from. Perhaps someone was trying to pee and accidentally soiled themselves. Afterwards they proclaimed, “I had to pee and…oops”. They took the P from p and the oops from the accident and the word poop was formed! Seriously, we got nothing, it was tough. Any other ideas?