imbecile

(offensive) A fool, an idiot.

(no longer used) A person with limited mental capacity who can perform tasks and think only like a young child, in medical circles meaning a person who lacks the capacity to develop beyond the mental age of a normal five- to seven-year-old child.

(offensive, old-fashioned) Destitute of strength, whether of body or mind; feeble; impotent; especially, mentally weak.