To mess with someone's mind. Often times, people will "psych you out" in preparation for a big game or competition by making you think that you don't stand a chance when put up against their superior skill.

Bart: Telegram for Lisa Simpson!

Homer: Don't listen, it's a trick!

Bart: Dear Lisa. Psych! Psych psych psych! Signed, Superpsych!

Homer: I think he's trying to psych you out...

Marge: We're all trying to have dinner, so why don't you just... PSYCH PSYCH PSYCH!

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