accusative singular neuter of appaṭivattiya, an adjective (likely misread or exceptional form; standard would be appaṭivattiyaṁ as accusative neuter singular)
Analysis. a- (not) + paṭi- (back, against) + vattiya (turned, from √vṛt turn). The compound negates the notion of turning back: that which does not retreat or reverse.
Etymology. The prefix a- (not) + paṭi- (back, facing) + vattiya (adj. of √vṛt 'turn', cognate with English 'wort' and Latin 'vertere'). The spatial imagery of paṭi- as 'turning back against' is negated, yielding the sense of irreversibility or non-return.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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