Праслав. *pěstъ, рус. пест и польск. Piast
The name of the founder of the Polish royal dynasty first appears in a Latin source as Past. There is no earlier polish evidence of its pronunciation. Historically it was read as Piast and considered to be derived from the continuant of Proto-Slavic *pěstъ ‘pestle’. However, Polish did not preserve this word, having stępor for ‘pestle’ and piasta for ‘axis’. H. Popowska-Taborska suggested that Piast derives from piastun ‘mentor’ pointing out the semantic grounds for such reconstruction. But absence of *piast meaning ‘mentor’ in Polish and an unusual word-building model prevented the scholar from accepting this model. Surprisingly, Russian dialectal data provide evidence for it. Old-rus. пестъ ‘a little child’ and rus. dial. пе́ста ‘an affectionate child’ demonstrate the objects of the action named in proto-slavic *pěstovati. The nomen agentis is *pěstunъ along with other non-object nouns: пест ‘bear’ and ‘ram’. Пест as ‘bear’ derives from пестун ‘a one-year-old bear cub’, semantics being close to ‘mentor’, as the eldest cub looks after the little ones. Пест ‘bear’ is a product of semantic generalisation accompanied by truncation.This is crucial for etymologisation of пест ‘ram-leader in herd’, as it is said to lead the herd to pastures without a shepherd. Bearing in mind the possibility of back formation and the semantics of leadership, it is likely that пест derives from пестун.The polish name Piast is likely to have undergone similar processes, though evidence from polish dialects or scripts is still desirable to be found.