imbecile

Meanings

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. A fool, an idiot.

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

Etymology

Borrowed from Middle French imbécile, from Latin imbēcillus (“weak, feeble”), literally “without a staff”.