Psycho

wiktionary

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Clipping of  psychotic and  psychopath +‎  -o(“person with characteristic”). 

Clipping of  psychology. 

etymonline

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psycho (adj.)

1927, shortening of psychological; 1936 (Raymond Chandler) as shortening of psychopathic (adj.).

psycho (n.)

1925 as short for psychologist; (earlier short for psychology, 1921); as short for psychopath by 1942.