A person with autism or some other mental disability who is exceptionally gifted in a highly specialized field, such as math (rapid mental calculation) or music (ability to play a complex piece of music after hearing it only once).
This term is now outdated. Autistic savant is the current term.

Etymology
From French, literally learned idiot. Usage
"I thought of him as a kind of idiot-savant - a man who had flashes of insight and brilliance. And he also had a great spirit." — Dan Harper; Save the Eccentrics; Santa Cruz Sentinel (California); Mar 21, 2004.

"The author, a self-proclaimed idiot savant, explains his motivation for the book on its back cover." — Drawing From Life; National Post (Canada); Mar 2, 2004.