ind. (instrumental ablative, from a feminine stem)
Analysis. pihā + ppaccayā; a dvandva or bahuvrīhi compound meaning 'desire-and-condition' or 'having desire as condition'. The doubling of p (< *paccayā with initial p-) shows the second member is treated as a separate unit. Renders the causal relationship 'because desire (is the) condition'.
Etymology. pihā (from √prih 'to love, wish'; Sanskrit prīh-, cognate with Latin precari 'to pray', English pray) + paccayā (from pati + i, 'going toward, falling to', thus 'condition, cause, support'). The whole phrase carries doctrinal weight: in Buddhist analysis, desire (taṇhā and related pihā) is identified as the root condition (paccaya) for suffering and rebirth.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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