demonstrative pronoun, nominative singular neuter, with metrical contraction and initial relative marker ya-
Analysis. ya- (relative marker) + idaṁ (demonstrative pronoun 'this'). The form yamidaṁ is a contraction used in metrical or emphatic contexts, often beginning a relative clause: 'that which is this', hence 'this very thing'.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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