imbecile

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   A fool, an idiot.

Adjective

1. 1
   Destitute of strength, whether of body or mind; feeble; impotent; especially, mentally weak.