Something that requires a lot of thinking or mental effort; often used in phrases like 'no-brainer' to mean something very simple.
Agent noun from 'brain,' adding the suffix '-er' (one who does something). In modern usage, a 'brainer' is something that demands brain activity or lacks the need for it.
The phrase 'no-brainer' is a fascinating inversion—it means something so obvious you don't need to use your brain, but it emphasizes the *absence* of thinking rather than its presence. It's a 20th-century expression reflecting modern impatience.
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