Where does “Πόντος Εὔξεινος” come from?

Πόντος Εὔξεινος (Ancient Greek) comes from Ancient Greek Πόντος Ἄξεινος, from Ancient Greek ἄξεινος, from Ancient Greek ἀ-, from Proto-Hellenic hə-, from Proto-Indo-European sm̥-, from Proto-Indo-European sem- — together, one.