As I said, it's a dialectical variant that's been around for some time. (And, if you're interested in such things, the etymology of the "true" word, "fr*g", dates back to the 15th century; it's from a late Middle English word signifying "to quiver", and was originally used to mean to move about randomly; rub -- usage about 1425-75).
Thanks for the etymology, JD - I'm writing a book set in London in 1593, so I'm always on the lookout for good period swear-words. The Online Etymology Dictionary dates its use as a euphemism to 1598, but of course that refers only to written evidence, so I'm sure it was in use long before then. I have a book on my shelves that's devoted purely to "rude" words in Shakespeare ("Shakespeare's Bawdy", by Eric Partridge) - the number of euphemisms for sex and the sex trade is amazing, and some phrases ("on the game") are still in use today...
In my book, mostly I use Shakespearian-style insults ("thou mewling whey-faced aleworm"*), or bits of British English slang that don't sound too modern, like
b-ll-cks. The F-word may be traceable as far back as the early 16th century in written records, but it does sound rather modern owing to the fact that it was banned in print until recent decades. OTOH it doesn't feel out of place in "The Lies of Locke Lamora", which has a quasi-historical setting that seems to me to fit somewhere between the 15th and 18th centuries, judging by the costume descriptions and overall level of technology. Since that seems to be the period when "swive", the main slang verb for copulation in the Middle Ages, was replaced by f---, that would make sense. If I do use f---, though, it certainly won't be in such abundance as in "Lies..."!
* the 'thou' is an important part of the insult, not a casual archaism - by the late-16th/early-17th century, it had become a by-word for name-calling:
"[SIZE=-1]If thou thouest him some thrice, it shall not be amiss" (Shakespeare,
Twelfth Night) - Sir Toby Belch, advising Sir Andrew Aguecheek on how to proceed in a duel, by beginnng with insults.[/SIZE]