Green’s Dictionary of Slang

freak-out n.

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

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

[US]P. Wylie Esquire July 44–45: freakout—a bad LSD experience.
[UK]J. Barlow Burden of Proof 150: They want pot. They need acid. They’re proud of taking a trip even if it’s a freakout.
[US]Rolling Stone 22 Sept. 57: The onstage freak-out brought on by dope and who knows what else.
[UK]M. Amis Experience 153: He experimented [...] with LSD. To me he seemed to be on the verge of total freakout for several hours.

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

[US]H.S. Thompson letter 28 Aug. in Proud Highway (1997) 637: This caused a general freak-out among local merchants who fear for the tourist trade.
[US]T. Southern Blue Movie (1974) 218: The instant Angela went into her freak-out, Fred the First had summoned the company physician.
[US]S. King Misery (1988) 212: He was edging towards a state of terminal freak-out.
[UK]I. Welsh Trainspotting 158: Sensin a freak oot, he sortay lets the lassie go.
[UK]T. Blacker Kill Your Darlings 41: Ned [...] who, after some horrific mid-life freak-out, now lived a tranquil, and probably tranquillized, life as a handyman.

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

[US]H.S. Thompson letter 21 Mar. in Proud Highway (1997) 607: I should be back in San Francisco for the summer freak-out by sometime in June.
[UK]M. Novotny Kings Road 98: We had the wildest freak-out last night.
[UK]K. Sampson Powder 27: The Grams were enjoying the sort of bacchanalian freakout usually the preserve of mushied-up drongos invading Stonehenge for the Solstice.

4. an orgy.

‘Groupie Gloss.’ on A. Lorber Groupies [album] Freak scene: sexual orgy.
[US]O. Hawkins Chili 41: It was scheduled to be a freak out. We were all going to get buck naked and fuck ourselves into bad health.

5. an explosion of temper.

[Ire]P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 19: So the old pair have a major freak out, and we’re talking major.