idiot

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A foolish or stupid person.
2. n. 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.

Century Dictionary and Cyclopedia

1. n. A private person.
2. n. An unlearned, ignorant, or simple person.
3. n. A fool or dupe; one who is fooled.
4. n. A professional fool; a jester; a clown.
5. n. A human being destitute of the ordinary mental powers; one who is born without understanding or discernment, or who has utterly lost it by disease, so as to have no lucid intervals; one who, by deficiency of the intellectual faculties, is unfit for the social condition, or for taking care of himself in danger.
6. n. In old English law, one who has been without understanding or reasoning powers from his birth, as distinguished from a lunatic. “At the present day idiocy is considered as a species of insanity or lunacy.” (Rapalje and Lawrence.)
7. Afflicted with or indicating idiocy; idiotic.
8. To make or render idiotic.

Wiktionary

1. n. pejorative A common term for a person of low general intelligence.
2. n. obsolete A medical or psychological term meaning a person who lacks the capacity to develop beyond the mental age of a normal four-year-old.

GNU Webster's 1913

1. n. obsolete A man in private station, as distinguished from one holding a public office.
2. n. obsolete An unlearned, ignorant, or simple person, as distinguished from the educated; an ignoramus.
3. n. A human being destitute of the ordinary intellectual powers, whether congenital, developmental, or accidental; commonly, a person without understanding from birth; a natural fool. In a former classification of mentally retarded people, idiot designated a person whose adult level of intelligence was equivalent to that of a three-year old or younger; this corresponded with an I.Q. level of approximately 25 or less.
4. n. A fool; a simpleton; -- a term of reproach.

WordNet 3.0

1. n. a person of subnormal intelligence