Green’s Dictionary of Slang

(orig. US) a notional ‘club’ of those who have enjoyed sex in an aeroplane. This has now been joined by the mile-deep club, those who have had sex while travelling through the Channel Tunnel between UK and France; thus mile-high n. an act of intercourse in an airplane.

1966		T. Keyes All Night Stand 152: The reporters had been going on about this mile-high club.
1992		D. Jarman diary 6 Aug. Smiling in Slow Motion (2000) 184: I asked the young man if he had joined the ‘mile high club’.
1996		P. Temple Bad Debts (2012) [ebook] ‘Jetsetting again,’ the driver said. ‘You joined that Mile High Club yet?’.
2006		Sam and the City at blogs.smh.com.au 20 June [blog] Yet we can’t talk about sex in public places without the mention of the Mile High Club (or MHC as it’s known to members).
2010		C. Hiaasen Star Island (2011) 72: ‘Was that your first mile-high’ Cherry asked.
2012		‘Dutch’ ? (Pronounced Que) [ebook] She [...] initiated him into the mile-high club all the way to Macau.