The headword is future optative, third person singular, of a(marta/nw (cf. alpha 1493, alpha 1494). This form is used by George of Pisidia (de expeditione Persica 1.137-8): after mentioning "the second Pharaoh" he appends - "if one might not err in calling second him who truly was first into error" (ei) deu/tero/n tis ou)x a(marth/soi le/gwn | to\n w(s a)lhqw=s prw=ton ei)s a(marti/an). In classical Greek the future optative would not have been used in the protasis of a conditional statement unless it was a subordinate clause in indirect discourse (Smyth §2287, web address 1); e)a/n with the subjunctive would be a normal construction. The lexicographer seems to be criticizing George's hyper-Attic grammar, but unfortunately the form he supplies instead is future indicative rather than aorist subjunctive (which would be a(marth/sh|). Perhaps this is merely a textual corruption. In any case, all three forms would have been pronounced the same in Byzantine times. As Tartaglia notes (80, n.29), "the second Pharaoh" gestures at the Persian king Chosroes II (cf. chi 418 note).