In normal society, slag is a derogatory term meaning frequently promiscuous and immoral. Typically a woman described as a slag might also be described as easy or cheap. It is strongly associated with women who, without consideration of there partner of the moment, seeks sexual gratification. The word is very offensive to women.
In the swinging scene, where there are tiers of sexuality, the word is associated with high sexual status, and is therefore coveted. Remarkably, as it is so offensive in everyday use, it is often used during intercourse as a compliment in recognition of sexual skill.
A girl who loses her partner to another woman might describe her as a slag.
A woman of exceptional sexual talent might be described as a slag within the swinging scene.