My Summer Guy: A guy who is just a summer fling. You generally start dating at the beginning of summer and break up at the end. Usually it is with a guy who is on vacation and will have to leave at the end of summer.
I went out with this totally hot guy during summer. He was definately M.S.G.
A gang in boston. it stands for Maple Street Gang
Yo dat nigga is from maple street aka m.s.g.