be-in, n. — Green’s Dictionary of Slang

be-in n.

[play on SE be in (touch)/being; taken from the original Human Be-in at Golden Gate Park, San Francisco, 1967; the -in sfx extended to incl. fuck-in, smoke-in, love-in, sit-in etc]

(orig. US) a gathering of young people, usu. hippies, for mutual admiration, smoking cannabis and listening to music.

1967		Oz 4 25: Just been to a very beautiful inter-planetary be-in on Venus.
1969		Guardian Weekly 20 Dec. 4: What, the Judge wanted to know, was a ‘be-in’. [...] Ginsberg explained it was ‘a gathering of young people imbued with a new planetary life-style’.
1978		A. Maupin Tales of the City (1984) 41: The acid, the music, the sex, the Avalon, the Family Dog, the Human Be-in.
1999		Guardian Rev. 26 June 9: Tom Wolfe’s classic essays on the ‘be-ins’ of the Sixties.
2000		T. Blacker Kill Your Darlings 24: Jimi had never been to an orgy before. Be-ins, love-ins, sit-ins, lie-ins but never an actual orgy.