Green’s Dictionary of Slang

a lover, orig. female but from mid-20C male, from a lower class and tougher background than their partner.

1977		Spare Rib Dec. 36/2: In this play a bit of fluff comes home to her bit of rough after being raped by her ex-husband at an arty-party.
1988		A. Hollinghurst Swimming-Pool Library (1998) 196: A bit of rough, you might say.
1989		T. Blacker Fixx 189: Then she could go mad [...] fall for her bit of rough.
1989		M. Amis London Fields 112: Keith knew that some of these rich ladies liked a bit of rough.
1999		Indep. Rev. 16 July 7: The traditional posh girl falling for a bit of rough.
1999		K. Sampson Powder 20: They’re howling for a bit of rough, this lot!
2000		J.J. Connolly Layer Cake 56: They make a strange couple, the lady and the bitta rough.
2004		Guardian G2 28 Dec. 14: She runs away to shack up with her new bit of rough in the dodgy area where he lives.
2017		L. McInerney Blood Miracles 132: ‘You’ve got a thing for a bit of rough [...] a thing for scumbags’.