An abbreviation for the instance "Scarlet Monastery", a dungeon in the game World of Warcraft. Usually used by player looking for people to group with. The instance is separated into four parts, the Library, the Armory, the Graveyard, and the Cathedral.
Hey, let's get a group and do SM!
LFG PST SM ARMORY!
Stupid noob, you are too much of a lowbie to do SM!