Green’s Dictionary of Slang

belly bump v.

also belly bust

to have sexual intercourse; one of a number of terms relating to the proximity of stomachs during the act; thus belly-bumping n. [note the saucily punning ballad ‘The Maiden’s Choice’ (1755): ‘And the tune that he plays is called belly pat’].

[UK]‘Walter’ My Secret Life (1966) IV 658: She might have done a little belly-bumping business by introductions.
[UK]Farmer Vocabula Amatoria (1966) 54: Caracoler: To copulate; ‘to belly-bump’.
[US]R.H. Rimmer Harrad Experiment 99: I service her regularly on Tuesday and Thursday [...] This schedule leaves me in shape for my own preference in belly bumping on Fridays and Saturdays.
[US]D. Lypchuk ‘A dirty little story’ in eye mag. 8 July [Internet] She had been belly-busted by a real Bluebeard. It was enough to run a girl into a bean-curd stirrer.
[UK]J.J. Connolly Viva La Madness 102: The incredibly good chances of indulging in belly-bumping sessions with extremely fit upper-class women.