This is from Herodotos
ὡς δὲ κατὰ νώτου ἐγένετο ἰούσης τῆς γυναικός ἐς τὴν κοίτην, ὑπεκδὺς ἐχώρεε ἔξω, καὶ ἡ γυνὴ ἐπορᾷ μιν ἐξιόντα.
I know that ὑπεκδύς is the aorist participle of ὑπεκδύομαι but only because Steadman tells me so.
It doesn't look like a participle to me.
So why does the participle look so odd and are there other verbs that behave in the same way?
Or am I missing something that ...