This is a noun used to describe a man with certain characteristics. These include mental and physical strength, the overwhelming desire to do the best he can in everything he does, including giving pleasure to his sexual partner, an over-sized penis and testicles, the ability to drink massive quantities of beer, and the total lack of fear in most situations.
"That guy Tony is a badass."
"Tony stole my girlfriend twice!"
"I heard one time Tony kicked Chuck Norris' ass!"