To intimidate or unnerve someone; an exclamation meaning 'just kidding' or expressing excitement.
Shortened from 'psychology' or 'psyche' (Greek psūkhē meaning 'soul' or 'mind'). The slang usage emerged in 1960s American English, originally meaning to psychologically manipulate, later becoming an exclamation of deception or excitement.
The word's evolution from ancient Greek philosophy to modern slang shows how academic terms become democratized. 'Psych!' as an exclamation preserves the original meaning of mind manipulation - you're literally telling someone you just played with their psychology.
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