How offensive is it to call someone a "slag" in British English? (NSFW)
Before getting to the main answer, a few points that may be interesting to you (though I realise they're not actually part of your question!):
- In the UK - at least in some circles, the word cunt is an insulting word for a man, never a woman: neither I nor those that I've asked have ever heard it used that way, though as the comments below attest, this usage does exist.
- Of the others, the odd-one-out to me would seem to be bitch: I'm not sure about US usage of that word, but to me the others all have a connotation (to a greater or lesser extent) of sexual impropriety. In the UK at least, bitch doesn't have that association - it means something along the lines of woman with an unkind, evil or spiteful personality.
- The words skank and ho are recognised - especially from rap music - but are definitely felt to be American imports. All of the others are fully accepted as native :)
On to the main question: it's probably worth distinguishing between the degrees of offensiveness of (abusing terminology slightly) use and mention of the words: that is, some words can be happily used in polite society, but one would be highly offended to be called one - whereas with some words merely saying the word (regardless of who it is applied to) is offensive in of itself.
Having drawn that distinction: mention of the word slag - while it's clearly to be avoided when on one's best behaviour - is unlikely to cause much offence; that would put it on a roughly similar level to slut or perhaps skank.
However, using the word slag to describe someone would be a good way to start a fight. It is probably at a similar level to slut: perhaps slightly more offensive, if only because slutty/sluttish can refer to general demeanour (in dress, speech) - and indeed can occasionally merely mean "untidy, slovenly" with no sexual connotation - whereas slag is unambiguously referring to sexual behaviour.
It's worth mentioning the word is used a lot in comedy - perhaps the earliest was in the comic Viz (decidedly not a comic for children, and never noted for its reverence); here's an example in Harry Enfield's sketch show; and more recently in Gavin and Stacey. (Using the word for men isn't common except in reference to this show!). You can probably see - especially from this last example - that this word can be thrown around among close friends in a humorous way - but I would strongly advise against doing so unless you're very sure of what you're doing, since the line between banter and offence is a very narrow one at times :)
It sounds pretty confrontational and insulting, and is certainly disparaging, if not downright offensive.
Etymology here:
slag - loose woman or treacherous man - the common association is with slag meaning the dross which separates during the metal ore (typically iron) smelting process. In fact the iron smelting connection is probably more of a reinforcing influence rather than an originating root of the expression. Francis Grose's Vulgar Tongue 1785 dictionary of Buckish Slang and Pickpocket Eloquence has the entry: "Slag - A slack-mettled fellow, not ready to resent an affront." In other words a coward. In this sense 'slack-mettled' meant weak-willed - combining slack meaning lazy, slow or lax, from Old English slaec, found in Beowulf, 725AD, from ancient Indo-European slegos, meaning loose; and mettle meaning courage or disposition, being an early alternative spelling of metal from around 1500-1700, used metaphorically to mean the character or emotional substance of a person, as the word mettle continues to do today. Partridge says that the modern slag insulting meaning is a corruption and shortening of slack-mettled. Certainly the associations between slack, loose, lazy, cheating, untrustworthy, etc., are logical. The mettle part coincidentally relates to the metal smelting theory, although far earlier than recent 20th century English usage, in which the word slag derives from clear German etymology via words including slagge, schlacke, schlacken, all meaning metal ore waste, (and which relate to the coal-dust waste word slack), in turn from Old High German slahan, meaning to strike and to slay, which referred to the hammering and forging when separating the waste fragments from the metal. Slag was recorded meaning a cowardly or treacherous or villainous man first in the late 18th century; Grose's entry proves it was in common use in 1785. Slag meaning a female prostitute seems to have first developed much later - around the 1950s - and its more general application to loose girls or women is later still, 1960s probably at soonest. So the notion that slag came directly from the iron and steel industry to the loose woman meaning is rather an over-simplification. The first slags were men, when the meaning was weak-willed and untrustworthy, and it is this meaning and heritage that initially underpinned the word's transfer to the fairer sex.
Slag is probably between bitch and cunt on your spectrum. It's not a word someone would use casually. You would only use it to describe someone that you really dislike and even then other words are to be preferred.
If you directly refer to someone as a slag then yes, you should prepare for a fight. My mental image of a situation where ''slag'' is used involves two groups of chavvy women, drunk, outside a pub, late at night, fighting over someone's boyfriend. That's pretty much the only time you should use the term.