(verb)
(informal) to intimidate someone (or make oneself intimidated) or make them feel uneasy in their mind, typically in advance of a challenge (used as "psych out");
e.g. I don't want to psych myself out too much by thinking about my opponent's strength.
(verb)
to correctly guess or anticipate someone's actions or intentions, or to examine something or figure it out using psychology (used as "psych out");
e.g. They tried to psych out the process so they would know what to expect.
(verb)
to investigate the way in which someone's mind works, to subject someone to psychoanalysis or psychotherapy;
e.g. They want to psych her to find the root of what is bothering her.
(abbreviation)
(informal) psychology or psychiatry, or a psychologist or a psychiatrist;
e.g. She wants to specialize in psych and become a mental health specialist.