imbecile

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.

A fool, an idiot.

Destitute of strength, whether of body or mind; feeble; impotent; especially, mentally weak.

“hospitals for the imbecile and insane”