Where does “kʷú só” come from?

kʷú só (Proto-Indo-European) comes from Proto-Indo-European kʷís, from Proto-Indo-European kʷi- — interrogative base.

Ancestry of “kʷú só”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	kʷís	who, what; who, which, that; which, what
2	Proto-Indo-European	kʷi-	interrogative base

kʷú só — Etymology, Origin & Word History | Etymology Explorer