A slag in Britain is a woman of loose morals ...

As in "You slag!"

oft shouted by the gilfriend of an unfaithful man at the object of his waywardness.

Or indeed by the boyfriend of an unfaithful woman

Nowadays used more affectionately between friends ...

"you fancy him don't you ... you slag ... "

And in this context is used to describe men also - manslag.

As is typical with the unfair nature of gender relations, for a lad, to be called a slag is something to grin about and wear like a medal, while for a lass it tarnishes her reputation, though with the 'laddicisation' (new word describing the emergence of the ladette in british culture ... you saw it here first) of women in Britain, it is being redefined as a badge of honour for girls too.

Ultimately though, it has lost none of it's power as an insult which suggests that promiscuity it describes still carries a social and cultural stigma.

A slag ... a wanton and promiscuous person.