"excrement," 1744, a children's euphemism, probably of imitative origin. The verb in this sense is from 1903, but the same word in the sense "to break wind softly" is attested from 1721; earlier "to make a short blast on a horn" (poupen, late 14c.). Meaning "stupid or dull person" is from 1915, but this is perhaps short for nincompoop. Pooper-scooper is attested from 1970.
Others are reading
<a href="https://www.etymonline.com/word/poo">Etymology of poo by etymonline</a>
Harper, D. (n.d.). Etymology of poo. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/poo