neuter nominative singular (idaṁ), with the particle etaṁ appended; the surface form idametaṁ appears to be a contraction or sandhi variant
Analysis. idam (neuter demonstrative pronoun, nominative singular) + etaṁ (accusative singular, emphasizing or anaphoric particle); the combination emphasizes the proximal or present referent, often appearing in discourse to mark what is being introduced or discussed.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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