Mouldy, torn bit of flannel used post-coitally by dirty slappers for 'mopping up' various copious liquids after incidents of explosive sexual union. Used predominantly by a Samantha (hehe).
Having blown him off but miscalculating the arc of his sexual effusions, Mary reached for her slag rag and wiped the residual bollock-chutney from her cheeks
Pass us the slag rag, will you? There's a good lad.
Jemima: "gosh, what I really need now is a slag rag"
<Goebbels passes Jemima a slag rag>
Jemima: "Thanks, Goebbels"
No, I said use the hand towel to dry your face, not the slag rag!