Green’s Dictionary of Slang

freak-out n.

also freak
[freak out v.]
[1960s+] (orig. US drugs)

1. any unpleasant experience caused by drug use, esp. with LSD.

2. anxiety, ranging from twinges of fear to a full nervous breakdown, varying as to context.

3. a gathering of young people, esp. hippies, to enjoy music and take drugs together.

4. an orgy.

5. an explosion of temper.