IMBECILITY

n. mentally handicapped condition (Psychology); stupidity, foolishness, silliness

Mental retardation is a term for a pattern of persistently slow learning of basic motor and language skills ("milestones") during childhood, and a significantly below-normal global intellectual capacity as an adult. One common criterion for diagnosis of mental retardation is a tested intelligence quotient (IQ) of 70 or below and deficits in adaptive functioning.
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Imbecility a state of mind in which an animal does feeble, weak, absurd things and does not respond to usual environmental stimuli. [more]Imbecility - Community and Resources

Noun
1. retardation more severe than a moron but not as severe as an idiot
(hypernym) retardation, mental retardation, backwardness, slowness, subnormality
2. a stupid mistake
(synonym) stupidity, betise, folly, foolishness
(hypernym) mistake, error, fault

(n.)
The quality of being imbecile; weakness; feebleness, esp. of mind.