From fuckipedia
False etymologies
One reason that the word 
fuck 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 recorded before the 1960s, according to the authoritative 
lexicographical work 
The F-Word, and thus are 
backronyms. In any event, the word 
fuck 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 
fuck came from 
Irish law. If a couple were caught committing 
adultery, they would be punished "For Unlawful 
Carnal Knowledge In the Nude", with "FUCKIN" written on the 
stocks above them to denote the crime. A similar variant on this theory involves the recording by church clerks of the crime of Forbidden Use of Carnal Knowledge. Another theory is that of a royal permission. During the 
Black Death in the 
Middle Ages, towns were trying to control populations and their interactions. Since uncontaminated resources were scarce, many towns required permission to have children. Hence, the legend goes, that couples that were having children were required to first obtain royal permission (usually from a local magistrate or lord) and then place a sign somewhere visible from the road in their home that said "
Fornicating Under Consent of King", which was later shortened to "FUCK". This story is hard to document, but has persisted in oral and literary traditions for many years; however, it has been demonstrated to be an urban legend.
[10]
 
Its first known use as a verb meaning to have sexual intercourse is in "
Flen flyys", written around 
1475.
William Dunbar's 1503 poem "Brash of Wowing" includes the lines: "Yit be his feiris he wald haue fukkit: / Ye brek my hairt, my bony ane" (ll. 13–14).
John Florio's 1598 Italian-English dictionary, 
A Worlde of Wordes, included the term, along with several now-archaic, but then vulgar synonyms, in this definition:
- Fottere: To jape, to sard, to fucke, to swive, to occupy.
Of these, "occupy" and "jape" still survive as verbs, though with less profane meanings, while "sard" was a descendant of the Anglo-Saxon verb 
seordan (or 
seorđan, <
ON serđa), to copulate; and "swive" had derived from earlier 
swīfan, to revolve i.e. to swivel (compare modern-day "screw").
While 
Shakespeare never used the term explicitly; he hinted at it in comic scenes in a few plays. 
The Merry Wives of Windsor (IV.i) contains the expression 
focative case (see 
vocative case). In 
Henry V (IV.iv), Pistol threatens to 
firk (strike) a soldier, a 
euphemism for 
fuck.