A person who, when consuming alcoholic beverages, is given to highly aggressive and antisocial behavior. This can range from simple verbal intimidation to physical assault. Mean drunks tend to overreact to small slights, and threaten physical violence toward anyone they perceive as threatening them. The term also implies that the person may be entirely normal when not drunk, and that it is the alcohol which causes the aggressive behavior. Contrast with a "happy drunk," who becomes much more social and frindly under the influence of alcohol.
Mean drunks appear to be a minority of the drinking population, and due to their aggressive behavior are not often welcome at social gatherings for long. Since they behave irrationally and often attack with little provocation, the best tactic is to stay out of their way.
"He's such a nice guy usually, but he's a mean drunk. Don't get in his way at a bar."