ἀλλ’ οὐ μὴ εἴπητε φρένας ἔχοντες
No, you will not speak with any sense!
The aorist subjunctive, preceded by οὐ μή, signals a strong negation in the future, both in the main clause and in subordinate clauses.
Since in the original construction μή was a subordinating conjunction, only the negation οὐ can be replaced by a negative adverb or pronoun (οὐκέτι, οὐδείς etc.).
The present subjunctive is also found, although much less frequently than the aorist.
This is a typically Attic construction. Historically the double negation may be explained as a case of insubordination, i.e. in which an (obligatory) subordinate clause becomes a main clause through ellipsis of the main verb: οὐ [φόβος ἐστι] μὴ φαῦλον ἦ ‘[There is no fear] that it is stupid.’
οὐ γάρ σε μὴ γήρᾳ τε καὶ χρόνῳ μακρῷ | γνῶσ’, οὐδ’ ὑποπτεύσουσιν, ὧδ’ ἠνθισμένον
For they will certainly not recognise you, because of your old age and the long period of time, and they will not suspect you thus adorned with white hair.
ἂν δ’ αὐτοὺς οἱ Τρῶες μὴ ἀποκτείνωσιν, οὐ μὴ ἀποθάνωσιν;
And if the Trojans do not kill them [the Greeks], will they really not die?