"For Unlawful Carnal Knowledge" was the official charge brought against prostitutes - of course, lazy law clerks would abbreviate the charge to f.u.c.k. - ing was later added to change the charge into a verb.
Once when a mighty knight was honor by the King, the King would give the knight his choice of a maiden. When the knight took his maiden to his room, the King would post a sign on the door "F.U.C.K." which meant "Fornicating Under Consent of King."
One of those made up acronyms that means Forcination Under Consent of the King, which was to be posted on the doors of those lower class couples in some hella old English period that wished to engage in sexual intercourse outside of the sanctity of marriage or will to conceive.