Likewise in Pausanias the Atticist epsilon11 and other lexica (also post-Suda, twice in Eusthatius' Commentary on Homer's Iliad). The headword (here nominative plural) is unattested outside lexicography.
Beyond the unexceptionable etymological point made, the substantive meaning of the headword would depend, in context, upon what ethnos meant there: people, nation, tribe, whatever; cf. generally epsilon 327.