A good friend. In paticular a good left-wing friend. Taken from the slang term "Broseph", for brother, or close associate. This is combined with Stalin, in reference to Joseph Stalin, the former leader of the USSR(Soviet Russia).
Guy 1: "Hey there man, what's up?"
Guy 2: "Not much, how about yerself there, Broseph Stalin?"
Guy 1: "Not much dog, not much"