Abbreviation for "Roast Beef Dangler." Used most commonly to ridicule someone regarding their overall level of uselessness. Derivations and alternatives include the noun "Dangler" or the adjective / verb "dangling." Can be modified to include any number of new forms, such as dangle-box, etc.
"What are you up to, R.B.D.?"
"Hey dangler, you wanna go to the bar tonight?"
"When are you going to get your dangling ass over here so we can leave?"