More commonly abbreviated to wmw, this is used to show that you think the person you are talking to is so low in soceity (a mcdonalds worker) their opinion doesn't matter. Most commonly used at a secondary school level. Can also be used with a hand gesture making a W with both your thumbs and index fingers, then turning it upside down for an M and then back up for a W.
When you didn't get anything to eat because you were not hungry, then upon your friend's food arrival you suddenly become hungry. This is known as the Mcdonalds Effect.
At first Brennan wasn't hungry, then when everyone else got their food the Mcdonalds Effect set in.