Where does “ваший” come from?

ваший (Ukrainian) comes from Ukrainian ваш, from Proto-Slavic vašь, from Proto-Indo-European túh₂ — you, thou.

Ancestry of “ваший”, step by step

StepLanguageWordMeaning
1Ukrainianвашyour, yours
2Proto-Slavicvašьyour
3Proto-Indo-Europeantúh₂you, thou
Every word from Proto-Indo-European túh₂ →Every word from Proto-Slavic vašь →Every word from Ukrainian ваш →