ENGLISH
REFERENCE

psycho

n. · adj.
US //ˈsaɪkoʊ// UK //sˈaɪkəʊ// psy·cho
  1. 1 a dangerous or unstable person (n.)
    B2 Upper Intermediate Informal Slang

    a person who acts in a very strange, violent, or scary way.

    a person perceived as being mentally unstable, aggressive, or dangerously unpredictable.

    Example

    Stay away from him; he acts like a total psycho when he gets angry.

    Example

    The tabloid media was quick to label the suspect a psycho, focusing on his erratic behaviour rather than the underlying social causes of the incident.

    Teacher's tip

    This word is often considered offensive or insensitive because it trivialises mental health issues; use it with caution.
  2. 2 crazy or scary (adj.)
    C1 Advanced Informal Slang

    behaving in a way that is crazy, scary, or out of control.

    exhibiting behaviour that is perceived as psychotic, irrational, or dangerously unstable.

    Example

    She has a psycho ex-boyfriend who keeps calling her from different numbers.

    Example

    His psycho reaction to a minor traffic disagreement left the other drivers in the car park feeling genuinely threatened.
  3. 3 psychology class (n.)
    C1 Advanced Informal American English

    a short way of saying 'psychology' when talking about a school or university subject.

    an informal, uncountable shorthand for the academic discipline or a specific course in psychology.

    Example

    I have my psycho 101 exam tomorrow morning, so I need to study tonight.

    Example

    Most students in the psycho department found the introductory lectures on behavioural patterns surprisingly challenging.

    Usage

    This sense is typically uncountable and used primarily in student contexts.
Etymology 1

Clipping of psychotic or psychopath + -o (“person with characteristic”). Compare sicko.

Etymology 2

Clipping of psychology.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

Last updated June 20, 2026