Where does “osrъ” come from?

osrъ (Proto-Slavic) comes from Proto-Balto-Slavic aśras, from Proto-Indo-European *h₂eḱrós, from Proto-Indo-European h₂eḱ- — sharp.

Ancestry of “osrъ”, step by step

StepLanguageWordMeaning
1Proto-Balto-Slavicaśrassharp
2Proto-Indo-European*h₂eḱróssharp
3Proto-Indo-Europeanh₂eḱ-sharp
Every word from Proto-Indo-European h₂eḱ- →Every word from Proto-Indo-European *h₂eḱrós →Every word from Proto-Balto-Slavic aśras →