Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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