imbecility imbecility /ˌɪmbəˈsɪlɪti/

noun

1. A stupid mistake  ( betise , foolishness , imbecility , folly , stupidity , bêtise )
   a stupid mistake
   
2. Retardation more severe than a moron but not as severe as an idiot  ( imbecility )
   retardation more severe than a moron but not as severe as an idiot