Inherited from Proto-Mongolic *biraxu, thought to be a borrowing from Proto-Turkic *buŕagu (“calf”). Compare Mongghul buruu, Middle Mongol بورو (/⁠burū⁠/), Old Turkic 𐰉𐰆𐰕𐰍𐰆 (buzaɣu), Turkish buzağı, Hungarian borjú.