indeclinable (locative adverb)
Analysis. tat- (that; neuter nominative/accusative singular of demonstrative tad) + -tha (locative suffix, also seen in ettha "here")
Etymology. From the demonstrative stem ta- (that), cognate with Sanskrit tat, Greek to (article), Latin istud; the locative suffix -tha appears in related forms like ettha and idhā. The demonstrative root extends back to Indo-European *to-, the ancestor of English "the", Greek "to", and Latin "istud".
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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