Definitions for no-brainerˈnoʊˈbreɪ nər
Random House Webster's College Dictionary
Informal.anything requiring little thought or effort; something easy or simple to understand or do.
Origin of no-brainer:
anything that requires little thought
Kernerman English Learner's Dictionary
no-brainer(noun)ˈnoʊˌbɒd i, -ˌbʌd i, -bə di
sth that is so obvious you should not need to ask or think about it
Doctors say that testing for these cancers should be a no-brainer.
An easy or obvious conclusion, decision, solution, task, etc.; something requiring little or no thought.
If the newer version performs as well for half the cost, the decision is a no-brainer.