penis.

Traditionally compared with Ancient Greek πέος (péos), Sanskrit पस॑स् (pásas), Old English fæsl (“offspring”) and derived from Proto-Italic *peznis, from Proto-Indo-European *pes-ni-s, from *pes- (“penis”). De Vaan is skeptical of this in view of the attested chronology of the word's semantics, where “tail” is older than “penis”, which also aligns with the expected semantic development of such a word (compare e.g. Old Armenian ձետ (jet, “tail”) > Middle Armenian ձետ (jet, “penis”)); the reverse development is less likely. Instead, he reconstructs a Proto-Italic form *petsnis (“tail”) and connects the word to penna (“wing; feather”).