εὐπίστως

From LSJ

Τί γὰρ γένοιτ' ἂν ἕλκος μεῖζονφίλος κακός; → What wound is greater than a false friend?

Sophocles, Antigone, 651-2

Russian (Dvoretsky)

εὐπίστως: с доверием, доверчиво (εὐ. ἔχειν Arph. - v. l. εὐπιθῶς).