Very intelligent or smart; using or requiring a lot of thinking and mental ability.
From 'brain' plus '-y' (suffix meaning having the quality of), a straightforward descriptive word popularized in American English in the late 1800s for intellectually gifted people.
Brainy became popular slang exactly when IQ testing started in the 1920s—we needed a friendly word for smart people that didn't sound snobbish, so we made it an adjective just like 'brainy scientist'!
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