Well, it does sound a bit like either "old sport" (Gatsby style) or mafia speech, because you wouldn't usually call your friend "my friend".
But if you want to sound rude without the risk of being misunderstood, I recommend using "Freundchen" (diminuitive) instead: "Get that dog off my lawn, Freundchen! (...or else!)"
Isn't deminuitive sounds even more rude than "my friend"? It seems to me that "my friend" is just sounds too official and ... like in the books or something like that? *I am not very good in English, but using deminuitivies to a friends sounds to me a bit superior. Like it is addressed to a dog, not to a human o_O correct me if I am wrong.