Where does “воистину” come from?

воистину (Russian) comes from Russian во-, from Russian в-, from Old Church Slavonic въ-, from Proto-Slavic vъ(n)-, from Proto-Indo-European h₁én — in.

воистину (Russian): truly, in truth, indeed, verily

Definitions

truly, in truth, indeed, verily

Ancestry of “воистину”, step by step

Step	Language	Word	Meaning
1	Russian	во-	Alternative form of в-
2	Russian	в-	in, into
3	Old Church Slavonic	въ-	in, into describing an action of moving inside of something
4	Proto-Slavic	vъ(n)-	—
5	Proto-Indo-European	h₁én	in

Every word from Proto-Indo-European h₁én →Every word from Russian в- →Every word from Russian во- →