Derivational etymology which is a variant of Herodian's etymology by ἔθος (see ἑταῖρος / ἔθος). It starts from a derivative of ἔθος which already has an [a]. The interesting point is the derivation: ἐθάς inflects ἐθάδος, but since only the nominative is taken into account for the etymology, the assumed change of the consonant from aspirate to non aspirate yields a ghost-form *ἔτας with no genitive attached to it. That enables the etymologist to deduce that the genitive is ἐτάρος, with a different inflection type from the etymon. This form indeed exists, as the Homeric variant of ἑταῖρος. Hence the logical conclusion (by Greek standards) that ἕταρος, and the derived form ἑταῖρος used in Ionic, is in fact a genitive turned into a nominative. It was not unusual for Greek etymologists to start from an inflected form, and in particular deriving the nominative of a given word from the homophonous genitive of another was not a problem: for instance the nominative κράτος "strength" was derived from the Homeric genitive of "head", κρατός (Choeroboscus, Epimerismi in Psalmos, p. 125: ἐκ τοῦ κράτος κράτερος κρατίων, τὸ ὑπερθετικὸν κράτιστος, παρὰ τὸ κάρα). See a comparable instance in ἀρήν / ἀρά.

This etymology derives ἑταῖρος from ἕταρος, whereas most others derive ἕταρος from ἑταῖρος.