A girl who is perverted, but still really sweet. The surprising combination of slut and muffin reflects the surprise when someone cute says something dirty.
A varient on the word slag, but not used as an exact synonym. Instead, it is an insult to someone who is being annoying, stupid or you just dislike.
(Playing poker)
Ben: "O.k so if I have a straight but with the ten, jack, queen, king, ace, is that a royal straght?"
Dudley "you fucking slag muffin."