It depends. When used towards a woman, it means that she is promiscuous. It's a pretty nasty insult. It can also sometimes be used as an insult to express contempt towards men. It has no sense of promiscuity. This use is more commonly heard in crime films and TV shows set in or around London.

Hello! This is a very nasty word. It means the same as a "slut". I would avoid using this word because it is never necessary to say this. I hope to have helped you!

The word "slag" is an English slang and it is a really nasty word. Not used by decent people.