Smooth Brain
An unintelligent person
Example
Can anyone help this smooth brain understand interest rates?
Smooth brain definition by Slang.net
This page explains what the slang term "Smooth brain" means. The definition, example, and related terms listed above have been written and compiled by the Slang.net team.
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People use smooth brain as a synonym for "idiot," "dummy," or "moron." This insult derives from the belief that the more folds a person's brain has, the smarter they are. So, the thinking goes, a person with a smooth brain is dumb as a rock.
You're most likely to encounter smooth brain on social media and in trashy Internet forums, in which people find it funny to mock each other's (lack of) intelligence. For example, the users of Reddit's r/wallstreetbets subreddit often use the term smooth brain (though this group often uses the term ironically).
However, being smooth-brained is a real medical condition, in which a child's brain develops without the folds normal brains have. Smooth-brained children often don't live past the age of 10. So, you may want to think twice before using the term smooth brain, online or IRL.