Also a verb, to
mad dog someone is to stare intently at someone as a means to
intimidate them. Often, the look conveys the message "don't you even
think about fuckin' wit' me, yo" or it could just be the harbinger for the introduction of the target to a "
world of hurt".
Its origins probably lie within the ghetto and gangspeak, wherein a culture of toughness is celebrated and worn like a badge, a passive and lesser form of assault and violence. As a customary practice, members of street gangs or residents of the hood keep large dogs and train them to sic on command, a veritable weapon of sorts. These dogs are often selected by their feral nature and are groomed as well as trained to appear as savage as possible. The canine equivalent of mad dogging occurs when the animal growls menacingly at its target, baring its teeth in preparation for a fight. The goal here is the same.
Not to be confused with a dirty look, which connotes an air of superiority of the subject, mad dogging someone says that you mean business. It doesn't mean you are better than the other person. Just tougher.
Being able to mad dog a potential combatant with minimal effort is an essential proficiency mastered by the seasoned bad ass.