Where does “Jesion” come from?

Jesion (Polish) comes from Proto-Slavic àsenь, from Proto-Indo-European Heh₃s- — ash tree.

Ancestry of “Jesion”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	àsenь	—
2	Proto-Indo-European	Heh₃s-	ash tree

Jesion — Etymology, Origin & Word History | Etymology Explorer