Thin Brain 

(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.

Thin Brain by Nubb'n December 17, 2008

Thin Brains 

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.

Thin Brains by jondich March 24, 2024