Something, especially a choice or decision, that is so obvious or easy as to require little or no thought.
Something so obvious, simple, etc. as to require little thought.
The definition of a no brainer is a situation in which there is one clear obvious or right answer and in which no real thought is required to decide what to do.
An example of a no brainer is if someone asks if you want a million dollars free and clear with absolutely no taxes or strings attached.