Wiktionary
idiot-savant
n. A mentally challenged person who is very gifted in one area of activity, e.g. the piano. This condition is associated with autism.
Usage examples of "idiot-savant".
However, his low intelligence quotient (moron), his extremely low general classification score, and his paranoid tendencies (delusions of grandeur) make it inadvisable to attempt to exploit his idiot-savant talent.