Proto-Indo-European Roots

Root/Stem: *kwis
Meanings:  who, what
Cognates:
Hellenic Greek tis 'who, what'
Italic Latin quis 'who', quid 'what', Oscan pis 'who', pid 'what'
Celtic Old Irish cid 'what'
Indic Sanskrit kis, kim 'who, what' 
Dardic & Nuristani
Iranian
Anatolian Hittite & Luwian & Palaic kuis 'who, what', Lydian qi- 'something, whatever', Lycian ti 'which'
Tocharian
Armenian
Balkan
Germanic
Baltic
Slavic Common Slavic *ki-to 'what' > 
Russian chto 'what'
Notes: This is one of the derivatives of the Indo-European root *kw-, which formed a great lot of pronouns in the languages of the family. English when, where, who, what, which and other pronouns of this kind are all descendants of this *kw-, the same as pronouns of other Indo-European languages. The root was mainly used for forming interrogative and relative pronouns.