[sahy-koh] Slang.
noun, plural psychos.
a psychopathic or psychotic person.
a crazy or mentally unstable person.
psychopathic or psychotic.
crazy; mentally unstable.

1935–40; by shortening Unabridged


a combining form representing psyche, (psychological ) and psychological, (psychoanalysis ) in compound words.
Also, especially before a vowel, psych-.

< Greek, combining form of psȳchḗ breath, spirit, soul, mind; akin to psȳ́chein to blow (see psykter) Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
psycho (ˈsaɪkəʊ)
n, —adj , pl -chos
See psychopath an informal and offensive word for psychopath or psychopathic

psycho- or (sometimes before a vowel) psych-
combining form
indicating the mind or psychological or mental processes: psychology; psychogenesis; psychosomatic
[from Greek psukhē spirit, breath]
psych- or (sometimes before a vowel) psych-
combining form
[from Greek psukhē spirit, breath]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1927 (adj.), shortening of psychological; 1936 as shortening of psychopathic (adj.). The noun , short for psychopath, is attested from 1942.

comb. form meaning "mind, mental," from Gk. psykho-, combining form of psykhe (see psyche).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

psycho psy·cho (sī'kō)
A psychopath. adj.
Crazy; insane.

psycho- or psych-

  1. Mind; mental: psychogenic.

  2. Mental activities or processes: psychomotor.

  3. Psychology; psychological: psychosocial.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Abbreviations & Acronyms
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
They'd rather have a rich neighbour than a crazy wacked out psycho next door
Nerds get what they want when they want it, and they go psycho if it's not
  immediately available.
Students and professors who experienced traumatic events raise the need for
  university psycho-social services.
Thus the yogi will distinctly come to realize the simultaneous occurrence in
  pair of these psycho-physical phenomena.
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