To forcibly eject someone from a business or house. Not escorting them out, not asking them politely to leave -- this is taking the guy and pitching him out the door and into the street. This is done either to someone who is making a big problem for the owners of the business or property or to someone who doesn't deserve the respect of being allowed to leave on their own.

The term derives from the practice of throwing homeless people or beggars out of restaurants or businesses, probably during the Depression, though the term may have appeared earlier. You didn't have to be nice to someone who didn't have the money or influence to hurt your business, so restaurant owners or bouncers were free to toss them out onto the sidewalk.