So I'm from America and the word slag isn't slang for anything here. after six(ish) months I finally bothered to look it up and in British terms it means "a woman who has many casual sexual encounters or relationships."
I honestly don't know what I was expecting given the context of the game. Maybe I just expected Ruth to be more socially aware of terms like that given her position or she just didn't spell it out in her head when giving the okay.