An individual who by all external perceptions appears to be a run of the mill dumb ass, and engages in correspondingly stupid behavior, only to BLOW YOUR FUCKING MIND when you actually see how well they preform in school with minimal effort. The idiot genius, when in a one-on-one conversation in a limited scope seems extremely intelligent, but their activities outside of that context remain in stark contradiction to their apparent intellect.
An Idiot Genius is the Anti-Intellectual Intellectual.
John: Dude, Mike pisses me off so much!
John: He does absolutely nothing in school and is a fucking straight A-Student! Then he gets trashed almost every day of the week, smokes his brain to mush, fucks girls like there's no tomorrow, and still manages to do better in school than me! What the FUUUUUCK?
Jim: Lol u mad bro? He's just an Idiot Genius!