The Collaborative International Dictionary of English v.0.48:

Amentia \A*men"ti*a\ ([.a]*m[e^]n"sh[i^]*[.a]), n. [L.] (Med.)
   Mental retardation. [archaic]
   [1913 Webster + AS]

WordNet (r) 3.0 (2006):

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