Slang now considered to be a common swear word

[Dukes01]

Where did the word F.U.C.K. come from.

[seniorgrumpy]

I thought that the CK referred to carnal knowledge but what the FU would be I don't know.

[LeMarchand]

The above seems to be an urban legend.

See

http://urbanlegends.about.com/library/bl-f-wor d.htm

..further details on the etymology here:

http://www.rampant-books.com/t_origin_f_word.h tm

[Quizmonster]

The earliest-recorded direct written use of the f-word in English was in a poem by the Scottish writer, William Dunbar, in 1503. There were earlier coy 'hints' about the word hidden away in written material. One of those was a satirical poem, written before 1500, that was about the Carmelite friars of Cambridge. A line in code reads, once it's �translated': "They are not in heaven because they f*** wives of Ely".
Some people claim it came from the German word 'ficken', meaning 'to strike' or 'to copulate with'. However, The Oxford English Dictionary - the etymology 'bible' - can see no reason to suppose there ever was any such direct connection and simply claims the etymology is unknown. It seems likely, however, that both words, the English and the German, may have had a common root in the ancient Germanic/Teutonic language.
Certainly there is no truth in the idiotic idea that the word is an acronym based on the phrases �Fornication Under Command (or Consent) of the King' or �For Unlawful Carnal Knowledge'.