idoneus.

Uncertain. Brugmann derives it from an unattested adverb *idō, like ultrōneus (“voluntary”) is from ultrō (“beyond”, adverb) and extrāneus (“foreign”) from extrā (“outside”, adverb). This adverb can be compared with Sanskrit इद् आ (íd ā́, “precisely that”) and univerbated इदा (idā́, “now”), Avestan 𐬌𐬜𐬀 (iδa, “now”), Gothic ita (“it”), and Old Irish ed (“it”), which Dunkel identifies with a sequence of Proto-Indo-European *íd (“it”) + *óh₁ (emphatic particle). Idōneus would therefore originally have meant something like “of such nature”. Walde and Hofmann prefer the analysis *idō < Proto-Indo-European *íd + *-dō, literally “to that” (compare dōnique). De Vaan gives no etymology.