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]

The Devil's Dictionary (1881-1906):

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


Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "imbecility":
   amentia, arrested development, asininity, backwardness, battiness,
   blithering idiocy, brainlessness, buffoonery, clownishness,
   crackpottedness, crankiness, craziness, cretinism, daffiness,
   desipience, disability, disablement, disqualification,
   eccentricity, fatuity, fatuousness, folly, foolery, foolheadedness,
   foolishness, frivolity, frivolousness, giddiness, goofiness,
   half-wittedness, idiocy, idiotism, inability, inadequacy, inanity,
   incapability, incapacitation, incapacity, incompetence,
   incompetency, inefficiency, ineptitude, infancy, infantilism,
   inferiority, insanity, insufficiency, legal incapacity, lunacy,
   madness, mental defectiveness, mental deficiency, mental handicap,
   mental retardation, mindlessness, minority, mongolianism,
   mongolism, mongoloid idiocy, moronism, moronity, niaiserie,
   nugacity, nuttiness, profound idiocy, queerness, retardation,
   retardment, sappiness, screwiness, senselessness, silliness,
   simple-wittedness, simplemindedness, simpleness, simplicity,
   stupidity, subnormality, thoughtlessness, triflingness, triviality,
   unfitness, wackiness, wardship, weirdness, witlessness, zaniness,
   zanyism