Green’s Dictionary of Slang

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

1965		P. Wylie Esquire July 44–45: freakout—a bad LSD experience.
1968		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.
1977		Rolling Stone 22 Sept. 57: The onstage freak-out brought on by dope and who knows what else.
2000		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.

1967		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.
1970		T. Southern Blue Movie (1974) 218: The instant Angela went into her freak-out, Fred the First had summoned the company physician.
1987		S. King Misery (1988) 212: He was edging towards a state of terminal freak-out.
1993		I. Welsh Trainspotting 158: Sensin a freak oot, he sortay lets the lassie go.
2000		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.
2016		T. Robinson Rough Trade [ebook] [H]e didn’t know whether to have himself a good old-fashioned freak out.

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

1967		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.
1971		M. Novotny Kings Road 98: We had the wildest freak-out last night.
1999		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.

1969		C. Brown Mr Jive-Ass Nigger 148: [I]f a chick wants a workout, I mean a freakout, that’s where they go. These Greeks work in teams, man. They fuck the chick between the toes, in the nose, and shit like that.
1969		‘Groupie Gloss.’ on A. Lorber Groupies [album] Freak scene: sexual orgy.
1985		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.

2000

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

6. (Aus.) any notably bizarre event, e.g. a black mass.

2000

R.G. Barrett Leaving Bondi (2013) [ebook] What a fuckin freak-out. [...] I’d have finished tied up on the altar with the May Queen as an offering to Zamzak.