This stands for shot gun noob
or at least i use it for that purpose I first learned it in Call of Duty 2 where everyone used it for someone who runs around shotgunning you(its really annoying). I then got Modern Warfare 2 and I still use it but I dont think other people do yet. But I dont know if anyone will tho.
Bob: That guy just came out of nowhere and shotguned me!
Joe:Yeh, he's a real sg noob