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]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "amentia":
   arrested development, backwardness, blithering idiocy, cretinism,
   half-wittedness, idiocy, idiotism, imbecility, infantilism,
   insanity, mental defectiveness, mental deficiency, mental handicap,
   mental retardation, mongolianism, mongolism, mongoloid idiocy,
   moronism, moronity, profound idiocy, retardation, retardment,
   simple-wittedness, simplemindedness, simpleness, simplicity,
   subnormality