Where does “въ-” come from?

въ- (Old East Slavic) comes from Proto-Slavic vъ(n)-, from Proto-Indo-European h₁én — in.

Ancestry of “въ-”, step by step

StepLanguageWordMeaning
1Proto-Slavicvъ(n)-—
2Proto-Indo-Europeanh₁énin
Every word from Proto-Indo-European h₁én →Every word from Proto-Slavic vъ(n)- →