The Collaborative International Dictionary of English v.0.48:

Idiot \Id"i*ot\ ([i^]d"[i^]*[o^]t), n. [F. idiot, L. idiota an
   uneducated, ignorant, ill-informed person, Gr. 'idiw`ths,
   also and orig., a private person, not holding public office,
   fr. 'i`dios proper, peculiar. See Idiom.]
   1. A man in private station, as distinguished from one
      holding a public office. [Obs.]
      [1913 Webster]

            St. Austin affirmed that the plain places of
            Scripture are sufficient to all laics, and all
            idiots or private persons.            --Jer. Taylor.
      [1913 Webster]

   2. An unlearned, ignorant, or simple person, as distinguished
      from the educated; an ignoramus. [Obs.]
      [1913 Webster]

            Christ was received of idiots, of the vulgar people,
            and of the simpler sort, while he was rejected,
            despised, and persecuted even to death by the high
            priests, lawyers, scribes, doctors, and rabbis. --C.
                                                  Blount.
      [1913 Webster]

   3. A human being destitute of the ordinary intellectual
      powers, whether congenital, developmental, or accidental;
      commonly, a person without understanding from birth; a
      natural fool. In a former classification of mentally
      retarded people, idiot designated a person whose adult
      level of intelligence was equivalent to that of a
      three-year old or younger; this corresponded with an I.Q.
      level of approximately 25 or less.
      [1913 Webster +PJC]

            Life . . . is a tale
            Told by an idiot, full of sound and fury,
            Signifying nothing.                   --Shak.
      [1913 Webster]

   4. A fool; a simpleton; -- a term of reproach.
      [1913 Webster]

            Weenest thou make an idiot of our dame? --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

idiot
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "idiot":
   alcoholic, aliene, alternating personality, ament,
   antisocial personality, ass, bedlamite, borderline case, born fool,
   clot, congenital idiot, crackbrain, crackpot, cretin, defective,
   dement, demoniac, disordered personality, disturbed personality,
   donkey, double personality, drug user, dual personality, dullard,
   dullhead, dumbbell, dummkopf, dummy,
   emotionally unstable personality, energumen, escapist, fanatic,
   flake, fou, golem, half-wit, hostile personality, hypochondriac,
   hypochondriast, ignoramus, imaginary invalid, imbecile,
   immature personality, inadequate personality, inferior personality,
   jackass, jerk, jester, juggins, kook, loon, loony, lunatic, madman,
   malade imaginaire, maladjusted personality, maniac,
   mentally defective personality, meshuggenah, mongoloid idiot,
   moral insanity, moron, motley, multiple personality, natural,
   natural idiot, natural-born fool, neuropath, neurotic,
   neurotic personality, nincompoop, ninny, noncompos, nut,
   paranoid personality, perverse personality, phrenetic,
   psychoneurotic, psychopath, psychopathic personality, psychotic,
   psychotic personality, raving lunatic, schizoid,
   schizoid personality, screwball, seclusive personality,
   sexual psychopath, shut-in personality, simp, simpleton, sociopath,
   split personality, stupid, tomfool, valetudinarian, valetudinary,
   weak personality, weirdo, zany



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

IDIOT, Persons. A person who has been without understanding from his
nativity, and whom the law, therefore, presumes never likely to attain any.
Shelf. on Lun. 2.
     2. It is an imbecility or sterility of mind, and not a perversion of
the understanding. Chit. Med. Jur. 345, 327, note s; 1 Russ. on Cr. 6; Bac.
Ab. h.t. A; Bro. Ab. h.t.; Co. Litt. 246, 247; 3 Mod. 44; 1 Vern. 16; 4
Rep. 126; 1 Bl. Com. 302. When a man cannot count or number twenty, nor tell
his father's or mother's name, nor how old he is, having been frequently
told of it, it is a fair presumption that, he is devoid of understanding. F.
N. B. 233. Vide 1 Dow, P. C. now series, 392; S. C. 3 Bligh, R. new series,
1. Persons born deaf, dumb, and blind, are, presumed to be idiots, for the
senses being the only inlets of knowledge, and these, the most important of
them, being closed, all ideas and associations belonging to them are totally
excluded from their minds. Co. Litt. 42 Shelf. on Lun. 3. But this is a mere
presumption, which, like most others, may be rebutted; and doubtless a
person born deaf, dumb, and blind, who could be taught to read and write,
would not be considered an idiot. A remarkable instance of such an one may
be found in the person of Laura Bridgman, who has been taught how to
converse and even to write. This young woman was, in the year 1848, at
school at South Boston. Vide Locke on Human Understanding, B. 2 c. 11, Sec.
12, 13; Ayliffe's Pand. 234; 4 Com. Dig. 610; 8 Com. Dig. 644.
     3. Idiots are incapable of committing crimes, or entering into
contracts. They cannot of course make a will; but they may acquire property
by descent.
     Vide, generally, 1 Dow's Parl. Cas. new series, 392; 3 Bligh's R. 1; 19
Ves. 286, 352, 353; Stock on the Law of Non Compotes Mentis; Bouv. Inst.
Index, h.t.



The Devil's Dictionary (1881-1906):

IDIOT, n.  A member of a large and powerful tribe whose influence in
human affairs has always been dominant and controlling.  The Idiot's
activity is not confined to any special field of thought or action,
but "pervades and regulates the whole."  He has the last word in
everything; his decision is unappealable.  He sets the fashions and
opinion of taste, dictates the limitations of speech and circumscribes
conduct with a dead-line.