kasmā

ka

masc abl sg of ka (interr); nt abl sg of ka (interr); masc abl sg of ka (pron); nt abl sg of ka (pron); …

ka (pron): who?; what?; which?
ka (pron): where?
ka (nt): water
kasmā (ind): why?; lit. from what?; from where?
ki (pron): who?; what?; which?

Built from

ka — root · who, what
Interrogative pronoun from *kwo- (Indo-European). Cognates: English who, Latin quis, Greek tís.
smā — ending · from, by (instrumental/ablative)
Instrumental or ablative singular marker. The localized case sense 'from what?' develops into the causal 'why?'.

Analysis. kasmā is the instrumental or ablative singular of ka (interr. pron. 'who?' 'what?'), literally 'from what?' or 'by what?', grammaticalized as an adverbial question-marker.

Etymology. ka is the interrogative pronoun (*kwo- in Indo-European), cognate with English 'who', Latin quis, Greek tís. The suffix -smā encodes the instrumental/ablative case, rendering the literal sense 'from/by what?' into the abstract 'why?' — a semantic shift common to interrogatives across Indo-European languages (compare Latin cūr 'why?', from the instrumental of qui).

Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.

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