Green’s Dictionary of Slang

(Aus.) of a man, to be one of the last to participate in the multiple rape of or intercourse with a woman.

1979		Lette & Carey Puberty Blues 76: Steve Strachan arrived late. ‘I’m not goin’ fuckin slops.’.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Slops. To be a late participant in a gang rape. As in ‘to go slops’.