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Wiktionary
idiot savant

n. A person who exhibits an extraordinary ability in one subject (often mathematics) whilst being mentally retarded in all other fields.

WordNet
idiot savant

n. person who is mentally retarded in general but who displays remarkable aptitude in some limited field (usually involving memory)

Usage examples of "idiot savant".

But he'd never heard of anyone changing from a normal person into an idiot savant.

However, it would have to be designed and promoted properly, and Somps, who struck me as something of an idiot savant, was certainly not the man for the job.

I had hundreds of megabytes stashed in my head on an idiot savant basis information I had no conscious access too.

And she learnt to use the spirit very skillfully, as her spy, her informant, her observer, her idiot savant adviser.

The ship's memory in the comlog had been an idiot savant, at best, but it had been with me for a long time.

Or, less charitably, charged that she was as much of an idiot savant as Boomer Petway.