feminine nominative, with iti (quotative marker)
Analysis. dhammadesanāya + iti: dhammadesanāya (instrumental singular of dhammadesanā) + iti (quotative particle 'thus, saying'). The phrase marks reported speech or introduces a saying about the Dhamma talk.
Etymology. dhammadesanā is a Sanskrit loanword compound: dhamma (Pali form of Sanskrit dharma, 'law, teaching') + desanā (from √dis 'to show, point out', cognate with Greek deiktikos 'showing'). The instrumental dhammadesanāya with iti is a standard frame for citation: 'by way of the Dhamma-teaching, thus'.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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