idiot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who is considered foolish or stupid.
• noun A person of profound mental retardation having a mental age below three years and generally 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.

from The Century Dictionary.

• To make or render idiotic.
• noun A private person.
• noun An unlearned, ignorant, or simple person.
• noun A fool or dupe; one who is fooled.
• noun A professional fool; a jester; a clown.
• noun 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.
• noun 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.)
• Afflicted with or indicating idiocy; idiotic.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete A man in private station, as distinguished from one holding a public office.
• noun obsolete An unlearned, ignorant, or simple person, as distinguished from the educated; an ignoramus.
• noun 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.
• noun A fool; a simpleton; -- a term of reproach.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pejorative A common term for a person of low general intelligence.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person of subnormal intelligence