Idiocy, n.
Etymology: [From idiot; cf. Gr. Idiot, and cf. Idiotcy.]
The condition or quality of being an idiot; absence, or marked deficiency, of sense and intelligence. I will undertake to convict a man of idiocy, if he can not see the proof that three angles of a triangle are equal to two right angles. F. W. Robertson.
Shades of meaning
Idiocy is a state of mental unsoundness amounting almost or quite to total absence of understanding. Imbecility is a condition of mental weakness, which may or may not be as complete as that of idiocy, but is at least such as to incapacitate for the serious duties of life. Incapacity, or lack of legal qualification for certain acts, necessarily results from imbecility, but may also result from other causes, as from insanity or from age, sex, etc.; as, the incapacity of a minor to make a contract. Idiocy or imbecility is weakness of mind, while insanity is disorder or abnormal action of mind. Folly and foolishness denote a want of mental and often of moral balance. Fatuity is sometimes used as equivalent to idiocy, but more frequently signifies conceited and excessive foolishness or folly. Stupidity is dulness and slowness of mental action which may range all the way from lack of normal readiness to absolute imbecility. Compare insanity.