nominative/accusative neuter singular + enclitic api; literally "water even" or "even water"
Analysis. udake (locative singular of udaka, "in water") + api (enclitic particle, "even, also"); forms an emphatic or concessive phrase "even in water" or "water too."
Etymology. udaka from the root √vad (Sanskrit vad-, Greek hydōr, English water, German Wasser) via Pali udar-/udk-, originally meaning "that which flows"; api from Indo-European *apo "off, away," cognate with Latin ab-, Greek apo-, English off — the sense "even/also" develops from the concessive force "and then, besides."
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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