Collins COBUILD Advanced Learner's English Dictionary - no-brainer

(no-brainers) 1. If you describe a question or decision as a no-brainer, you mean that it is a very easy one to answer or make. (AM INFORMAL) If it’s illegal for someone under 21 to drive, it should be illegal for them to drink and drive. That’s a no-brainer. N-COUNT 2. If you describe a person or action as a no-brainer, you mean that they are stupid. (AM INFORMAL) N-COUNT c darkgreen]disapproval