A term given to someone who doesn’t use so it goes unused/rusty.
This is used as an insult, basically calling someone dumb. Often used for people who value feelings and opinions over logic and reality.
Person 1 - “Does this novel have pictures in it?”
Person 2 - “bro, you have brain rust”
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Person 1 - “You can’t use the word ‘fat’ it’s offensive”
Person 2 - “I think you’ve got some brain rust bro”