a slag could be meant as a compliment as well though. i am often refered to as an old slag by my friends, it just means that i get around....

old slag = gigolo' or old mate or one that travels a lot!

note from the cockney dictionary:

slag(ed) (old slag) (slag off): tart, slapper, tarted, put it about, to old to get laid (old slag), talk derogatively about someone, (me ol' slag) my old friend goes on an' on