[This might be a better fit for the linguistics SE, but I'm especially skeptical of the German etymology, hence posting it here.]
The standard, ubiquitous and seemingly accepted etymology for German Popo S.m., meaning butt, ass, is given through studentensprachliches Latin podex, abbreviated and reduplicated to Popo. It is mainly, but not exclusively (and definitely not in the abbreviated Po with the same meaning), used when talking to childen, and the reduplication is considered as somehow related to children's language.
By itself, I find no reason to doubt this well-accepted derivation. However, I have now seen a multitude of very similar words with very similar meanings, in languages that are not at all similar to German, and moreover, I have come across several distinct etymologies, depending on the language.
Closest to German in this list, in English, we have poo and poo-poo, meaning not butt, but fecies, and according to etymonline, it is supposed to be of "imitative origin".
In Italian, we have popò, meaning again fecies, but Treccani claims it was modelled after pipí, meaning, lo and behold, pee.
In Latin Spanish, there is popó, which can mean both, depending on the country you're in, and apparently absent in Peninsular Spanish. I couldn't find an etymology.
Also in French, popo can mean both fecies and chamberpot, and at least Wiktionary says it is derived from pot.
In Hungarian (!), we have popó, meaning butt, in concordance with the German Popo; I couldn't find an etymology.
Of all these etymologies, I find those I couldn't find the most convincing. I sincerely doubt that the German etymology is correct, as it appears as the most contrived in my eyes. Is there any reason to believe it (in particular believe it above e.g. claiming its being modeled after German Pipi), beyond appeal to authority (such as Grimm, I understand)?