ive heard two theories of the F word's origen
1. Fornication Under Consent of the King (F.#.*.K)
the initials were posted above the first legal brothels in england ... but i cant remember which king legallised prostitution as long as it was "controlled" in a brothel
This, I believe, has been debunked in recent years as popular myth.
The word definitely has Anglo-Saxon origins, but beyond that, I have no clue.
LabRat