The Collaborative International Dictionary of English v.0.48:

Psycho- \Psy"cho-\
   A combining form from Gr. psychh` the soul, the mind, the
   understanding; as, psychology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

psycho
    n 1: a person afflicted with psychosis [syn: psychotic,
         psychotic person, psycho]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "psycho":
   abnormal, bereft of reason, brainsick, catatoniac,
   certifiable case, crackbrained, cracked, crazed, crazy, daft,
   deluded, demented, deprived of reason, deranged, disoriented,
   distraught, flighty, hallucinated, insane, irrational, loco,
   lunatic, mad, maddened, manic, manic-depressive, mazed, mental,
   mental case, mentally deficient, meshuggah, moon-struck,
   non compos, non compos mentis, not all there, not right, odd,
   of unsound mind, off, paranoiac, paranoid, psychopath,
   psychopathic case, psychopathic personality, psychotic, queer,
   reasonless, schiz, schizo, schizoid, schizophrenic, schizy,
   senseless, sick, stark-mad, stark-staring mad, strange, tetched,
   touched, unbalanced, unhinged, unsane, unsettled, unsound,
   wandering, witless