An abbreviation of: "shaking my rat", smr is typically used when something is obvious, plain old stupid, or in disappointment, and is used primarily by LBC_Bulletproof.
In Black Ops 2 it is a extremely overpowered gun in multiplayer but its absolutely dogshit in zombies needs to burn in a hole I say. DONT GET ME STARTED ON THE PACK A PUNCH VARIANT THE SMILR the only thing it makes smile is the zombies when you shoot them.