A facetious term coined by Morgan Spurlock in his documentary movie Super Size Me. It refers to the extra methane one gets from eating lots of McDonald's food. That extra flatulence harms people around you and also further damages our ozone layer, and is usually followed by a McBrick
Dude 1: "Yesterday I watched Super Size Me. That was some hilarious stuff Spurlock said while eating the quarter pounder meal."
Dude 2: "What did he say?"
Dude 1: "He goes: 'See, now's the time of the meal when you start getting the McStomach ache. You start getting the McTummy. You get the McGurgles in there. You get the McBrick, then you get the McStomach ache. Right now I've got some McGas that's rockin'.'
Dude 2: "ROFLMAO - McGas - that's McFuckedUP dude. LOL"