1. Being incredibly good at something, like fighting, sports, or more commonly video games.
2. A word that comes before an action to emphasize that doing so is incredibly difficult, nigh impossible. Can also be used to describe a past action that is amazing in nature and was performed with incredible skill and all around badassness.
1.That guy just kicked the entire bars ass single handed, if that's not beast, then I don't know what is.
2. Did you watch UFC yesterday? It was a beast fight man, neither of em would back down, looked like they were gonna go to the death!