http://www.straightdope.com/columns/read/1009/whats-the-origin-of-the-f-word
I always figured it was probably an age-old instance of Saxon onomatopoeia. 
(Just say it to yourself five times with the appropriate rhythm.)
My parents, like many white people, tragically lack soul. In an attempt to remedy this, I brought them a copy of The Commitments to watch one evening. Sadly, all they got out of it was that “fuck” seemed to be an onomatopoeia.
FWIW,
Rob
I offered this somewhat jokingly, but I really wonder. Clearly, the words burp, fart, scratch, suck are all onomatopoeia.
It goes back at least to proto-Germanic, since we can find cognate words in other Germanic languages. The cognate words seem to mean something like “to strike.” Whether this was onomatopoeia at some point is impossible to say. “Burp,” “fart,” and “suck” are apparently onomatopoeia, but they all go back to at least proto-Indo-European. When we say that a word is onomatopoeia, that doesn’t mean that it has to be recent onomatopoeia. There are a number of families of cognate words in Indo-European languages that were probably formed as onomatopoeia at some point in proto-Indo-European (or even earlier) times and which have continued to sound similar in all the later versions of these Indo-European languages. “Scratch” doesn’t go back to proto-Indo-European. It may have been onomatopoeia at one point, but it’s impossible to say for sure.