It is when you are so tired, drunk, or drugged, that you brain no longer works or filter out thought. You say whatever you think. No one knows what you are talking about, because your thoughts have no focus.
"Dude, what are you talking about. You must havedrunk brain. Go sleep it off."
The feeling you get after eating a burrito and taco from anywhere but Chipotle, then every picture of a chain restaurant looks like a Chipotle. Often associated as a form of food coma.
"Cate sent me a snapchat from a foodcourt and I had really bad Chipotle Brain."