This is the discount available as a reward for staying faithful to a prostitute. If you frequently visit the same whore, you might be offered a "slut discount" for your loyalty. Very rare, but from time to time it does happen.
Simon: I've been seeing the same prostitute for about 3 months now and last night she said that because I was a regular client, and loyal, I deserved a "slut discount". I now only have to pay £20 for the full works, it used to be £30.
Lee: Thats brilliant, what are you going to do with the money you save?
Simon. Well, its my wife's birthday soon, so I might take her out for a nice meal.
Lee: Your such a romantic, she is lucky to have you.
Simon: Yeah, I know.