disapproval If you describe a person or action as a no-brainer, you mean that they are stupid. a decision that is easy, and that you do not need to think about, used when you want to emphasize that it is really very easy
If you describe a question or decision as a no-brainer, you mean that it is a very easy one to answer or make. 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