demonstrative pronoun (instrumental singular masculine/neuter) + emphatic particle, appearing as a contraction or sandhi fusion in the surface form imināpāhaṁ (resolved: imināya + pāhaṁ or similar emphatic construction)
Analysis. iminā (instrumental of ima 'this') + pāha (emphatic particle akin to vā 'or, indeed', intensifying the demonstrative force). The fusion imināpāhaṁ = 'by/with this very [thing]', common in canonical exhortation and description of causal sequences.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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