From Longman Dictionary of Contemporary Englishbrain deadbrain deada) MIin a state where your brain has stopped working properly, even though your heart may still be beating b) STUPID/NOT INTELLIGENT informal in a state in which you seem stupid or uninteresting, especially because you live a boring life or are very tired → brain
Examples from the Corpus
brain dead• As laid back as you can get without being declared clinically brain dead.• But he was pronounced dead, you know the doctor came and said brain dead.• What's the matter with you? Are you brain dead or something? 