An old urban legend goes somethin' like this:
In olden times, when adultery was still illegal (or at least the laws enforced,) those caught in the act would be sentenced to a day in the pillory, with a large sign hung beneath that read: For Unlawful Carnal Knowledge.
So, according to myth, the acronym "FUCK" became a popular insult for adulterers, and eventually acquired the meaning it still has today.
Of course, this is utter nonsense. See the entry for fuck for a detailed etymology.
The acronym varies from legend to legend. Popular versions include Fornication Under Consent of the King and Fornication Under Christ, King.