Ultimately from Proto-Indo-European*pṓds. Compare the nominative/accusative dual form, paine, presumably from a Proto-Tocharian dual *peine (whence also Tocharian A nominative/accusative dual form peṃ), from an earlier *pei, from the Proto-Indo-European dual *póde, from *pṓds. It is from this dual form in Proto-Tocharian that the singular forms have probably been analogically built. Compare Tocharian A pe. [1]