The word origin urban myth goes like this...
"There was a day when those in England needed the permission of the King to fornicate. Once they recieved this permission, they would hang a sign outside stating "Fornicating Under Consent of the King." This was shortened to the convenient acronym F.U.C.K. over time. The practice of asking the King ended, but the term for the act remained."
For the facts, try this site:
http://www.wordorigins.org/
Enjoy, AR+