Wikipedia has everything: )
http://en.wikipedia.org/wiki/Fuck

There are several urban-legend fake etymologies postulating an acronymic origin for the word.

In the most popular version, it is said that the word "fuck" came from Irish law. If a couple committing adultery were "Found Under Carnal Knowledge" they would be penalized, with "FUCK" written on the stocks above them to denote the crime.
Alternative explanations for "fuck" as an acronym for adultery pin it as "Fornication Under Cardinal/Carnal Knowledge".
Another story is that it was written in the log book as "FUCK" when people in the military or navy who had homosexual intercourse were being punished. Variants of this include "For Unlawful Carnal Knowledge", "Felonious Use of Carnal Knowledge", "Full Unlawful Carnal Knowledge", and "Forced Unlawful Carnal Knowledge", a label supposedly applied to the crime of rape.
In another story, a sign reading "Fornication Under Consent of the King" was supposedly placed on signs above houses in medieval England during times of population control and was special permission given to knights, by their king, when a knight wished to have sex with a woman.