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]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMBECILITY, med. jur. A weakness of the mind, caused by the absence or
obliteration of natural or acquired ideas; or it is described to be an
abnormal deficiency either in those faculties which acquaint us with the
qualities and ordinary relations of things, or in those which furnish us
with the moral motives that regulate our relations and conduct towards our
fellow men. It is frequently attended with excessive activity. of one or
more of the animal propensities.
     2. Imbecility differs from idiocy in this, that the subjects of the
former possess some intellectual capacity, though inferior in degree to that
possessed by the great mass of mankind; while those of the latter are
utterly destitute of reason. Imbecility differs also from stupidity. (q.v.)
The former consists in a defect of the mind, which renders it unable to
examine the data presented to it by the senses, and therefrom to deduce the
correct judgment; that is, a defect of intensity, or reflective power. The
latter is occasioned by a want of intensity, or perceptive power.
     3. There are various degrees of this disease. It has been attempted to
classify the degrees of imbecility, but the careful observer of nature will
perhaps be soon satisfied that the shades of difference between one species
and another, are almost imperceptible. Ray, Med. Jur. ch. 3; 2 Beck, Med.
Jur. 550, 542; 1 Hagg. Ecc. R. 384; 2 Philm. R. 449; 1 Litt. R. 252, 5 John.
Ch. R. 161; 1 Litt. R. 101; Des Maladies mentales, considerees dans leurs
rapports avec la legislation civille et criminelle, 8; Georget, Discussion
medico-legale sur la folie, 140.



The Devil's Dictionary (1881-1906):

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