Stands for Let's Get This Money. Often reffered to as getting stuff done, doing something awesome such as beerbonging or shotgunning beers and or bench pressing insane weights, or housing crazy amounts of pot pies late in the night. Often shouted to rouse lazy party goers or others in the weight room.
Guy one: Dude I've already had 15 and ate burger king i dont kow if I can shotgun anymore...
Guy two: LGTM!!!!!
Guy one: I don't know dude, seriously my stomach hurts!
Guy two: LGTM!!!!!!!!!!!
Guy one: Alright dammit... lgtm