The feeling you get when you are having a lively discussion or listening to an engaging lecture and everything makes sense and it gives you a bunch of new ideas about the topic at hand. The intellectual equivalent of an erection.
DeSean: oh man, that lecture given by Dr. Kobilka about G protein-coupled receptor dynamics gave me a massive brain boner.
Derrick: uhh...right, yeah, totally dude. *pssst* Jimmy, I think we need to re-evaluate our friendship with DeSean...