To refer to someone who's gone to a level of insanity, (Usually from being treated like really terribly) That they become a psycho and may end up harming someone, the term got popular from the Postal game series
Guy 1: I'M GONNA KILL YOU, YOU SON OF A WHORE I WILL GUT YOU AND YOUR FAMILY!!!
A band created by Ben Gibbard and Jimmy Tamborello sending lyrics and rhythms back and forth through mail. Has the most crazy amazing lyrics ever created. Not to be confused with the US Postal Service.