I've not seen such a genre acronym before, and the feeling was that I hit a road block. Thinking it's probably some ero game, and the fella might be a closet perv, I hazarded a guess: Sexy Lady Game? Nope. The only thing I can think that is of suitable use for this acronym is: Super Lag Game.
Imagine during a crappy game of Halo:
P1: WTF...pwned by crappy head shot..^@^$#%$!#$@^%^&**(
P2: slg man....slg...
So, if you are faced with crappy play (on your part, or someone else's) in an online/multiplayer game, take a deep breath and repeat after me: "slg man.....slg.."