"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. Idiocy (1487) is perhaps modeled on prophet/prophecy.
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.