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!