Sexual arousal that comes from enjoying someone else intelligence.
Guy1: Have you seen that girl? She is incredibly intelligent.
Guy2: Yeah, I would go over an horde of blood thirsty aliens to have her. I have an incredible Brain Boner.
Girl1: I can' t belive the way he explained Heisenber uncertainty principle.
Girl2: Yeah, he gave me a Brain Boner since the start of the class.