A filthy "Punk" "whore"
Either way punk slut is an insult to someone who may or may not be a slut.
Just like punk bitch, Punkslut basically has the same meaning, but, Sounds odd or funny when said.
dude 2: weak...
Dude 1: Man, I had the best romp last night with Lucy.
Dude 2: Oh rly? well I heard she was a punkslut, better check for std's dude.
dude 3: oh...