The phrase refers to the often explosive bowel moments caused by the use of Mass Gainer supplements and used as a code word.
*Note these guys used 8-12 scoops of Mass Gainer a day, however they also trained very intensely and made significant gains. Drinking Mass Gainers without a proper workout regiment will mostly result in many MGMs and belly fat as well.
man 1: Where are you going?
man 2: I have to take an MGM
Mass Gainer Moment- (aka MGM)
man 3: WTFs an MGM??
man 1: Another MGM?
man 2: ya! its my 5th one today...the story of my life...