The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Cruelty . . . argues not only a depravedness of nature,
         but also a meanness of courage and imbecility of mind.
                                                  --Sir W.
                                                  Temple.
   [1913 Webster]

   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.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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, imbecility]

The Devil's Dictionary (1881-1906):

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