Green’s Dictionary of Slang

1. a group gathering to express mutually loving feelings.

1967		Time 25 Aug. 46: Everybody was happy, sniffing the incense, smoking pot. It was a real love-in.
1971		Norman Mailer in Harper’s Mag. Mar. 44: A world of junkies, hippies, freaks, and freaks who made open love at love-ins, be-ins, concerts, happenings.
1989		D. Waters Heathers [film script] We’re just gonna write off today, and on Friday Pauline can hold her little ‘love-in’, or whatever.
1999		Guardian Guide 14–20 Aug. 59: An acid-drenched love-in.
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.

2. (also grope-in) an orgy.

1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 45: My name’s Roxana, and this is Arabella. Are we going to have a love-in?
1968		N. von Hoffman We are the People Our Parents Warned Us Against 155: There was no grope-in (group sex), but several couples were seen having relations on the Pentagon lawn.

3. positive, optimistic relations.

2000

Guardian Media 6 Mar. 2: After a tremendous love-in with the media.