Where does “v” come from?

v (Slovak) comes from Proto-Slavic vъ(n), from Proto-Indo-European h₁n̥, from Proto-Indo-European h₁én — in.

Ancestry of “v”, step by step

StepLanguageWordMeaning
1Proto-Slavicvъ(n)in, inside, within; in, into, inside; at
2Proto-Indo-Europeanh₁n̥—
3Proto-Indo-Europeanh₁énin
Every word from Proto-Indo-European h₁én →Every word from Proto-Indo-European h₁n̥ →Every word from Proto-Slavic vъ(n) →