pronoun, ablative singular + emphatic particle
Analysis. tamhā (ablative singular of ta 'that') + eva (emphatic particle 'indeed, just, alone'); the combination strengthens the sense of origin or isolation from a specific point.
Etymology. ta is the demonstrative pronoun cognate with Sanskrit tad, Greek to (definite article), Latin istud; the ablative suffix -hā (-ā in Sanskrit) marks 'from/out of'. eva is an old emphasizing adverb, perhaps related to Sanskrit eva and Greek ē 'or' in its emphatic sense.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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