Where does “weśredni” come from?

weśredni (Polish) comes from Polish średni, from Old Polish śrzedni, from Proto-Slavic *serdьňь.

Ancestry of “weśredni”, step by step

Step	Language	Word	Meaning
1	Polish	średni	medium; mid; average
2	Old Polish	śrzedni	middle
3	Proto-Slavic	*serdьňь	—