Where does “остро-” come from?

остро- (Russian) comes from Russian о́стрый, from Proto-Slavic ostrъ, from Proto-Balto-Slavic aśras, from Proto-Indo-European *h₂eḱrós, from Proto-Indo-European h₂eḱ- — sharp.

Ancestry of “остро-”, step by step

StepLanguageWordMeaning
1Russianо́стрыйpointed, sharp; sharp, pointed
2Proto-Slavicostrъsharp
3Proto-Balto-Slavicaśrassharp
4Proto-Indo-European*h₂eḱróssharp
5Proto-Indo-Europeanh₂eḱ-sharp
Every word from Proto-Indo-European h₂eḱ- →Every word from Proto-Indo-European *h₂eḱrós →Every word from Proto-Balto-Slavic aśras →