Definition of no-brainer noun from the Oxford Advanced Learner's Dictionary

 

no-brainer

 noun
noun
BrE
 
; NAmE
 
(informal)
 
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a decision or a problem that you do not need to think about much because it is obvious what you should do The question of who to support in this election should be a no-brainer. Providing a survivor benefit for a spouse is a no-brainer for most people.
See the Oxford Advanced American Dictionary entry: no-brainer