Søren Sørensen, who came up with the concept of pH, wasn’t clear on the mean of the letter “p”. It does involve powers of 10, which can also be measured with electrical potential, so the best guess are “potential” or “power”, or several words that mean “power” in other languages and also happen to start with “p”. Bottom line, we don’t know, and unless somebody discovers more of Sørensen’s notes or a way to speak with the dead, we never will.