imbecility

1. 1. Great stupidity or foolishness.
   2. Something, such as conduct or an act, that is stupid or foolish.
2. Psychology. The state or condition of being an imbecile; moderate or severe mental retardation.

noun

Foolish behavior: absurdity, folly, foolery, foolishness, idiocy, insanity, lunacy, madness, nonsense, preposterousness, senselessness, silliness, tomfoolery, zaniness. Informal craziness. See ability/inability.

A state of mind in which an animal does feeble, weak, absurd things and does not respond to usual environmental stimuli. Usually congenital. See also hydranencephaly, encephalopathy.

n.

A kind of divine inspiration, or sacred fire affecting censorious critics of this dictionary.

The noun has 2 meanings:

Meaning #1: retardation more severe than a moron but not as severe as an idiot

Meaning #2: a stupid mistake
  Synonyms: stupidity, betise, folly, foolishness