A bastardization of the greeting "How are you doing?" Used by the stereotypical mobster in popular culture. Heard in countless movies, TV shows, commercials, etc. Most often used in real life by actual mobsters, Americans of Italian descent living in New York or New Jersey (and elsewhere), or retards who want to sound tough.
Tony Soprano: (to his crew as he walks into the Bada Bing!) "How you doin'?"