early 14c., "person so mentally deficient as to be incapable of ordinary reasoning;" also in Middle English "simple man, uneducated person, layman" (late 14c.), from Old French idiote "uneducated or ignorant person" (12c.), from Latin idiota "ordinary person, layman; outsider," in Late Latin "uneducated or ignorant person," from Greek idiotes "layman, person lacking professional skill" (opposed to writer, soldier, skilled workman), literally "private person (as opposed to one taking part in public affairs)," used patronizingly for "ignorant person," from idios "one's own" (see idiom).
Reader, suppose you were an idiot. And suppose you were a member of Congress. But I repeat myself. [Mark Twain, c.1882]Idiot box "television set" is from 1959; idiot light "dashboard warning signal" is attested from 1968. Idiot savant attested by 1870.
idiot id·i·ot (ĭd'ē-ət)
A person of profound mental retardation having a mental age below three years and generally being unable to learn connected speech or guard against common dangers. The term belongs to a classification system no longer in use and is now considered offensive.