I'm trying to understand how the Greek word for backside/rear could be derived from the PIE word for 'to flow'. There is a Sanskrit word arsati which means 'to pierce', so the meaning of the PIE root is not in doubt. But I dont quite follow how "flow" could be the root word for "backside", as wiki (https://en.m.wiktionary.org/wiki/οὐρά#Ancient_Greek) claims (which I'm sure is valid too).
The idea is that there were two homophonic IE roots: *h₁ers- "tail" and *h₁ers- "to flow". Nobody is claiming that the two are connected.