The Collaborative International Dictionary of English v.0.48:

Idiocy \Id"i*o*cy\ ([i^]d"[i^]*[-o]*s[y^]), n. [From idiot; cf.
   Gr. ? uncouthness, lack of education, fr. ?. See Idiot, and
   cf. Idiotcy.]
   The condition or quality of being an idiot; absence, or
   marked deficiency, of sense and intelligence.
   [1913 Webster]

         I will undertake to convict a man of idiocy, if he can
         not see the proof that three angles of a triangle are
         equal to two right angles.               --F. W.
                                                  Robertson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

idiocy
    n 1: extreme mental retardation [syn: idiocy, amentia]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "idiocy":
   amentia, arrested development, asininity, backwardness, battiness,
   blithering idiocy, brainlessness, buffoonery, clownishness,
   crackpottedness, crankiness, craziness, cretinism, daffiness,
   desipience, eccentricity, fatuity, fatuousness, folly, foolery,
   foolheadedness, foolishness, frivolity, frivolousness, giddiness,
   goofiness, half-wittedness, idiotism, imbecility, inanity,
   ineptitude, infantilism, insanity, lunacy, madness,
   mental defectiveness, mental deficiency, mental handicap,
   mental retardation, mindlessness, 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, wackiness, weirdness,
   witlessness, zaniness, zanyism



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

IDIOCY, med. jur. That condition of mind, in which the reflective, or all or
a part of the affective powers, are either entirely wanting, or are
manifested to the least possible extent.

     2. Idiocy generally depends upon organic defects. The most striking
physical trait, and one seldom wanting, is the diminutive size of the head,
particularly of the anterior superior portions, indicating a deficiency of
the anterior lobes of the brain. According to Gall, whose observations on
this subject are entitled to great consideration, its circumference,
measured immediately over the orbiter arch, and the most prominent part of
the occipital bone, is between 11« and 14« inches. Gall, sur les Fonctions,
p. 329. In the intelligent adult, it usually measures from 21 to 22 inches.
Chit. Med. Jur. 248. See, on this subject, the learned work of Dr. Morton,
of Philadelphia*, entitled Crania Americana. The brain of an idiot equals
that of a new born infant; that is, about one-fourth, one-fifth, or one-
sixth of the cerebral mass of an adult's in the enjoyment of his faculties.
The above is the only constant character. observed in the heads of idiots.
In other respects their forms are as various as those of other persons. When
idiocy supervenes in early infancy, the head is sometime remarkable for
immense size. This unnatural enlargement arises from some kind of morbid
action preventing the development of the cerebral mass, and producing serous
cysts, dropsical effusions, and the like.
     3. In idiocy the features are irregular; the forehead low, retreating,
and narrowed to a point; the eyes are unsteady, and often squint the lips
are. thick, and the mouth is generally open; the gums are spongy, and the
teeth are defective; the limbs are crooked and feeble. The senses are
usually entirely wanting; many are deaf and dumb, or blind and others are
incapable of perceiving odors, and show little or no discrimination in their
food for want of taste. Their movements are constrained and awkward, they
walk badly, and easily fall, and are not less awkward with their hands,
dropping generally what is given to them. They are seldom able to articulate
beyond a few sounds. They are generally affected with rickets, epilepsy,
scrofula, or paralysis. Its subjects seldom live beyond the twenty-fifth
year, and are incurable, as there is natural deformity which cannot be
remedied. Vide Chit. Med. Jur. 345; Ray's Med. Jur. c. 2; 1 Beck's Med. Jur.
571 Shelf. on Lun. Index, h.t.; and Idiot.