Imbecile (noun)

1. A person with a mild to moderate intellectual disability.
2. A person who is extremely foolish or incompetent.

Origin:

From french "imbécile" meaning foolish or mentally deficient person.

Examples:

1. He was called an imbecile for his foolish decision.
2. The imbecile was unable to understand the instructions.
3. It is a cruel thing to call someone an imbecile.
4. The imbecile's lack of common sense was astounding.
5. The imbecile had no idea what he was doing.