(Noun)Derogatory name for a man with a small penis. Used after one shows their small penis at a party or gathering.
John was dared to streak across the bar. I don't know if it was just cold or if he was a thin brain.
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Someone who is unintelligent and/or cannot comprehend complex situations
He has thin brains becuase he cannot comprehend that Atlanta is more than a place on the map.