Green’s Dictionary of Slang

bun v.1

[? bun n.1 (2), i.e. the notoriously sex-driven SE bunny-rabbit]

1. (US black) to have sexual intercourse.

[US]G. Smitherman Black Talk.

2. (Aus.) to subject a woman, voluntarily or otherwise, to intercourse with multiple males.

[Aus]R.G. Barrett Mud Crab Boogie (2013) [ebook] The boys were in there bunning Warren’s girl.