Green’s Dictionary of Slang

(orig. UK services) an act of sexual intercourse.

1932		G.S. Moncrieff Café Bar 58: ‘You old sport,’ she said to me [...] ‘I’ll give you a bunk up for nothing.’.
1958		(con. 1940s) B. Behan Borstal Boy 173: ’E’s after ’aving ’is morning bunk-up.
1958		F. Norman Bang To Rights 187: I expect you’ll be having a bunk up this time tomorrow night?
1962		F. Norman Guntz 194: My rule about not bunging birds for bunk-ups.
1973		B.S. Johnson All Bull 93: The hope that [...] you could have your first ‘bunk-up’ with a girl.
1982		P. Bailey Eng. Madam 34: Word got round at the police station that Hamilton Payne’s daughter had been caught having a bunk-up in the garden shed.
1995		J. Cameron Vinnie Got Blown Away 45: Been heading for the bunk-up with Kelly eight hours ago.
2000		J.J. Connolly Layer Cake 192: I need a good charver, a bitta freestyle, a good bunk-up.
2006		D. Seabrook Jack of Jumps (2007) 240: A bender, in other words, with the chance of a bunk-up thrown in.