imbecile

Etymology

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

noun

1. (obsolete) 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.
   It is an offence for a man to have unlawful sexual intercourse with a woman whom he knows to be an idiot or imbecile. 1956, Parliament of the United Kingdom, “Part I, section 7”, in Sexual Offences Act 1956, page 2
2. (derogatory) A fool, an idiot.

adj

1. (dated) Destitute of strength, whether of body or mind; feeble; impotent; especially, mentally weak.
   hospitals for the imbecile and insane