The word "idiot" comes from the Greek ???????, idi?t?s 'a private person, individual', 'a private citizen' (as opposed to an official), 'a common man', 'a person lacking professional skill, layman', later 'unskilled', 'ignorant' from ?????, idios 'private', 'one's own'. In Latin, idiota was borrowed in the meaning 'uneducated', 'ignorant', 'common', and in Late Latin came to mean 'crude, illiterate, ignorant'. In French, it kept the meaning of 'illiterate', 'ignorant', and added the meaning 'stupid' in the 13th century. In English, it added the meaning 'mentally deficient' in the 14th century.

Many political commentators have interpreted the word "idiot" as reflecting the Ancient Greeks' attitudes to civic participation and private life, combining the ancient meaning of 'private citizen' with the modern meaning 'fool' to conclude that the Greeks used the word to say that it is selfish and foolish not to participate in public life. In fact, this is incorrect: though the Greeks did value civic participation and criticize non-participation, they did not use "idiot" to describe non-participants, or in a derogatory sense; its most common use was simply a private citizen or amateur as opposed to a government official, professional, or expert. The derogatory sense came centuries later, and was unrelated to the political meaning.

In 19th- and early 20th-century medicine and psychology, an "idiot" was a person with a very profound intellectual disability. In the early 1900s, Dr. Henry H. Goddard proposed a classification system for intellectual disability based on the Binet-Simon concept of mental age. Individuals with the lowest mental age level (less than three years) were identified as idiots; imbeciles had a mental age of three to seven years, and morons had a mental age of seven to ten years. The term "idiot" was used to refer to people having an IQ below 30^{[citation needed][12][13]} IQ, or intelligence quotient, was originally determined by dividing a person's mental age, as determined by standardized tests, by their actual age. The concept of mental age has fallen into disfavor, though, and IQ is now determined on the basis of statistical distributions.^[14]

In the obsolete medical classification (ICD-9, 1977), these people were said to have "profound mental retardation" or "profound mental subnormality" with IQ under 20.^[15] This term is not in use in the United Kingdom.

United States

Until 2007, the California Penal Code Section 26 stated that "Idiots" were one of six types of people who are not capable of committing crimes. In 2007 the code was amended to read "persons who are mentally incapacitated."^[16] In 2008, Iowa voters passed a measure replacing "idiot, or insane person" in the State's constitution with "person adjudged mentally incompetent."^[17]

In several U.S. states, "idiots" do not have the right to vote:

• Kentucky Section 145^[18]
• Mississippi Article 12, Section 241^[19]
• New Mexico Article VII, section 1^[20]
• Ohio Article V, Section 6^[21]

The constitution of the state of Arkansas was amended in the general election of 2008 to, among other things, repeal a provision (Article 3, Section 5) which had until its repeal prohibited "idiots or insane persons" from voting.^[22]

A few authors have used "idiot" characters in novels, plays and poetry. Often these characters are used to highlight or indicate something else (allegory). Examples of such usage are William Faulkner's The Sound and the Fury, Daphne du Maurier's Rebecca and William Wordsworth's The Idiot Boy. Idiot characters in literature are often confused with or subsumed within mad or lunatic characters. The most common intersection between these two categories of mental impairment occurs in the polemic surrounding Edmund from William Shakespeare's King Lear. In Fyodor Dostoevsky's novel The Idiot, the idiocy of the main character, Prince Lev Nikolaievich Myshkin, is attributed more to his honesty, trustfulness, kindness, and humility, than to a lack of intellectual ability. Nietzsche claimed, in his The Antichrist, that Jesus was an idiot. This resulted from his description of Jesus as having an aversion toward the material world.^[23]