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]