Where does “kaH” come from?

kaH (Proto-Iranian) comes from Proto-Indo-Iranian kás, from Proto-Indo-European kʷós — who.

Ancestry of “kaH”, step by step

StepLanguageWordMeaning
1Proto-Indo-Iraniankáswho
2Proto-Indo-Europeankʷóswho
Every word from Proto-Indo-European kʷós →Every word from Proto-Indo-Iranian kás →