There is a Latin word exostra with a Greek cognate ἐξώστρα, that enters mishnaic Hebrew as גזוזטרא, with the meaning balcony/enclosure.
I heard from a friend that Latin exostra derives from ex + sto, and that the Greek cognate ἐξώστρα derives from ἔξω + ἵστημι. This seemed fishy to me, so I looked up in dictionaries (on Perseus) that said that exostra derives from the Greek (the Greek dictionaries on Perseus that I saw gave no etymology). Then the Etymological Hebrew Dictionary by Haim Rabin suggests that ἐξώστρα derives from ἐξ + ὠθέω. Now, this etymology makes semantic sense to me, but I don't understand noun formation enough to identify where the suffix -στρα derives from. Is there such a nominal suffix? Is this the correct etymology?