The Collaborative International Dictionary of English v.0.48:

Non compos \Non com"pos\ Non compos mentis \Non com"pos men"tis\
   [L.]
   Not of sound mind; not having the regular use of reason;
   hence, also, as a noun, an idiot; a lunatic; one devoid of
   reason, either by nature or from accident.
   [1913 Webster]

WordNet (r) 3.0 (2006):

non compos mentis
    adj 1: not of sound mind, memory, or understanding; in law, not
           competent to go to trial [syn: non compos mentis(p),
           of unsound mind(p)]