1. something that is obvious and requires little thought
Professional
Social
the decision was a total no-brainer for me
This is generally used in relation to a choice or decision, where 'a no-brainer' is a plan or option that is believed to promise the best possible outcome. If the outcome of an investment is 100% guaranteed to return massive profits, then it's definitely a 'no-brainer' to choose to invest. This is an informal expression.