Source : Webster's Revised Unabridged Dictionary (1913)

Idiocy \Id"i*o*cy\, n. [From idiot; cf. Gr. ? uncouthness, want
   of education, fr. ?. See {Idiot}, and cf. {Idiotcy}.]
   The condition or quality of being an idiot; absence, or
   marked deficiency, of sense and intelligence.

         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.

Source : WordNet®

idiocy
     n : extreme mental retardation [syn: {amentia}]