what is bad-ass

bad-ass also have a positive meaning other than these negative meaning (n.
A mean-tempered or belligerent person; adj. Mean; belligerent

In the US, when you say someone or something is bad-ass it means doing something really really superior, and really good, an example will be:

John built a model, it was very detailed and nicely done. His classmates will say to John, "John, you are such a bad-ass when it comes to building models!" (meaning John is really good at building models)

bad·ass (băd'ăs') Vulgar Slang.

n.
A mean-tempered or belligerent person.

adj.
Mean; belligerent.