Learn English – What are the origins of using the abbreviation “v.” for “very”

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!):

1. 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.
2. 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.
3. 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 :)

I apologize in advance for not being able to provide a more well-documented answer, or even a whole answer, but I did find this.

From: Eureka Encyclopedia

Boy (Broad boy; Buff boy; Business boy; Call boy; Career boy; Come-on boy; Fag boy; Party boy; Pratt boy) Male prostitute.

it also lists

Rent Boy

and

Baggage Boy