ἀπομνύναι

From LSJ

οὐκ ἔστι λύπης ἄλγημα μεῖζονthere is no greater pain than grief

Source

Greek > English (Woodhouse Verbs Reversed)

(see also ἀπόμνυμι): deny by oath, deny on oath

Lexicon Thucydideum

iure iurando promittere, to promise under oath, 5.50.1.