Source : Webster's Revised Unabridged Dictionary (1913)

Imbecility \Im`be*cil"i*ty\, n.; pl. {Imbecilities}. [L.
   imbecillitas: cf. F. imb['e]cillit['e].]
   The quality of being imbecile; weakness; feebleness, esp. of
   mind.

         Cruelty . . . argues not only a depravedness of nature,
         but also a meanness of courage and imbecility of mind.
                                                  --Sir W.
                                                  Temple.

   Note: This term is used specifically to denote natural
         weakness of the mental faculties, affecting one's power
         to act reasonably or intelligently.

   Syn: Debility; infirmity; weakness; feebleness; impotence.
        See {Debility}.

Source : WordNet®

imbecility
     n 1: retardation more severe than a moron but not as severe as an
          idiot
     2: a stupid mistake [syn: {stupidity}, {betise}, {folly}, {foolishness}]