(negative connotation) A belligerent or mean person; a person with an unpleasantly extreme appearance, attitudes, or behavior.

"Don’t mess with that guy, he’s a real badass."

(youth slang, positive connotation) A person considered impressive due to courage, skill, and/or toughness.

(negative connotation) Belligerent and troublesome.

(positive connotation) Having an extreme appearance, attitude, or behavior that is considered admirable.

"That tough guy looks badass."