Where does “простительный” come from?

простительный (Russian) comes from Russian прости́ть, from Old East Slavic простити, from Proto-Slavic prostiti.

Ancestry of “простительный”, step by step

Step	Language	Word	Meaning
1	Russian	прости́ть	—
2	Old East Slavic	простити	—
3	Proto-Slavic	prostiti	—

Words derived from “простительный”

Every word from Proto-Slavic prostiti →

простительный — Etymology, Origin & Word History | Etymology Explorer