[Toro]

BS on F.U.C.K.

False etymologies
One reason that the word [censored] is so hard to trace etymologically is that it was used far more extensively in common speech than in easily traceable written forms.

There are several urban-legend false etymologies postulating an acronymic origin for the word. None of these acronyms was ever heard before the 1960s, according to the authoritative lexicographical work, The F-Word, and thus are backronyms. In any event, the word [censored] has been in use far too long for some of these supposed origins to be possible. Some of these urban legends are:

That the word [censored] came from Irish law. If a couple were caught committing adultery, they would be punished "For Unlawful Carnal Knowledge In the Nude", with "[censored]" written on the stocks above them to denote the crime.
That it came from any of:
"Fornication Under Carnal/Cardinal Knowledge"
"Fornication Under [the] Control/Consent/Command of the King"
"Fornication Under the Christian King"
"False Use of Carnal Knowledge"
"Felonious Use of Carnal Knowledge"
"Felonious Unlawful Carnal Knowledge"
"Full-On Unlawful Carnal Knowledge"
"For Unlawful Carnal Knowledge"
"Found Under Carnal Knowledge"
"Forced Unlawful Carnal Knowledge", referring to the crime of rape.