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]