Someone who is really intelligent, but doesn't have their head on straight. They have a thirst for knowledge and could've done anything with their life, but ended up doing nothing. They can chat politics or philosophies all night long over a beer, but aren't motivated to apply their knowledge toward a career. They are insanely smart in weird ways and know crazy amounts of trivia. You always want them on your team in board games. In the game of life they are self-sabotaging.
He's like Matt Damon in Good Will Hunting: the smartest guy I've ever known, yet he's a janitor. He's a mentallectual.