Slag is a derogatory slang term used in the UK, Ireland and Australia to describe most British women. Deriving from the term as used for the ugly piles (slag heaps) of impurities during the smelting of metals the term achieved a wider user with the comic strip "The Fat Slags" in Viz magazine.
Although the term can be compared with slut, whereas a slut may be of either sex and any level of beauty, slag is specifically female and ugly. Slag would also never be used as a positive term about oneself.