idiot

Etymology

From Middle English idiote, ydiote, from Old French idiote (later idiot), from Latin idiota, from Ancient Greek ἰδιώτης (idiṓtēs, “a private citizen, one who has no professional knowledge, layman”), from ἴδιος (ídios, “one's own, pertaining to oneself, private”). Doublet of idiota.

noun

1. (derogatory) A person of low general intelligence.
2. (derogatory) A person who makes stupid decisions; a fool.
   We think that people who cycle without a helmet are idiots.
3. (obsolete, medicine, psychology) A person of the lowest intellectual standing, a person who lacks the capacity to develop beyond the mental age of a normal four-year-old; a person with an IQ below 30.
   It is an offence for a man to have unlawful sexual intercourse with a woman whom he knows to be an idiot or imbecile. 1956, Parliament of the United Kingdom, “Part I, section 7”, in Sexual Offences Act 1956, page 2

adj

1. (uncommon) idiotic, stupid