Where does “восьмы” come from?

восьмы (Belarusian) comes from Proto-Slavic osmъ, from Proto-Balto-Slavic aśmas, from Proto-Indo-European oḱtṓwos, from Proto-Indo-European oḱtṓw, from Proto-Indo-European aḱ- — sharp, pointed; sharp, pointed, edgy; sharp,...

восьмы (Belarusian): eighth