Heavily intoxicated by drugs or alcohol, especially to the point of becoming unconscious, nonsensical, or out of control. Primarily heard in UK. I think someone might have spiked Jack's drink with some kind of drug, because he's out of his brain all of a sudden. Jane is a lightweight. Only one or two beers gets totally out of her brain. I had to take my brother away from the party because he was out of his brain on LSD.