Where does “вашь” come from?

вашь (Old East Slavic) comes from Proto-Slavic vašь, from Proto-Indo-European túh₂ — you, thou.

Ancestry of “вашь”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	vašь	your
2	Proto-Indo-European	túh₂	you, thou

вашь — Etymology, Origin & Word History | Etymology Explorer