sandhi contraction: yad + atthi
Analysis. yad (relative pronoun, nominative singular neuter, 'which/what') + atthi (3rd person singular present of √as, 'is'). The contraction is purely phonetic: yad + atthi → yadatthi by elision of the initial vowel of atthi. Idiomatically, 'whatever is' or 'as many as exist'.
Etymology. yad from Proto-Indo-European *kʷod (interrogative-relative stem, cognate with Latin quod, English what); atthi from √as 'to be', cognate with Sanskrit asti, Latin est, English is. The sense 'whatever there is' flows from the relative-indefinite force of yad ('which/what') applied to the copula.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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