Where does “plej” come from?

plej (Esperanto) comes from Latin plūs, from Old Latin plous, from Proto-Indo-European ploh₁is-.

plej (Esperanto): most

Definitions

Step	Language	Word	Meaning
1	Latin	plūs	—
2	Old Latin	plous	—
3	Proto-Indo-European	ploh₁is-	—

plej — Etymology, Origin & Word History | Etymology Explorer