Popular etymologies agree, unfortunately incorrectly, that this is an acronym meaning either Fornication Under Consent of the King or For Unlawful Carnal Knowledge. The latter usually accompanying a story about how medieval prisoners were forced to wear this word on their clothing.
Deriving the etymology of this word is difficult, as it has been under a taboo for most of its existence and citations are rare. The earliest known use, according to American Heritage and Lighter, predates 1500 and is from a poem written in a mix of Latin and English and entitled 'Flen flyys.' The relevant line reads:
"Non sunt in celi quia fuccant uuiuys of heli."
Translated:
"They [the monks] are not in heaven because they fuck the wives of Ely [a town near Cambridge]."
Fuccant is a pseudo-Latin word and in the original it is written in cipher to further disguise it.
Ayto mentions the word's use in 1278 as a personal name, John le Fucker , but fails to provide a citation. The earliest usage cite in the OED2 dates from 1503 and is in the form 'fukkit.' The earliest cite of the current spelling is from 1535.
The word was not in common (published) use prior to the 1960s. Shakespeare did not use it, although he did hint at it for comic effect. In Merry Wives of Windsor (IV.i) he gives us the pun "focative case." In Henry V (IV.iv), the character Pistol threatens to "firk" a French soldier, a word meaning "to strike," but commonly used as an Elizabethan euphemism for fuck. In the same play (III.iv), Princess Katherine confuses the English words "foot" and "gown" for the French "foutre" and "coun" (fuck and cunt, respectively) with comic results. Other poets did use the word, although it was far from common. Robert Burns, for example, used it in an unpublished manuscript.
The taboo was so strong that for 170 years, from 1795 to 1965, fuck did not appear in a single dictionary of the English language. In 1948, the publishers of The Naked and the Dead persuaded Norman Mailer to use the euphemism "fug" instead, resulting in Dorothy Parker's comment upon meeting Mailer: "So you're the man who can't spell fuck."
The root is undoubtedly Germanic, as it has cognates in other Northern European languages: Middle Dutch fokken meaning to thrust, to copulate with; dialectical Norwegian fukka meaning to copulate; and dialectical Swedish focka meaning to strike, push, copulate, and fock meaning penis. Both French and Italian have similar words, foutre and fottere respectively. These derive from the Latin futuere.
While these cognates exist, they are probably not the source of fuck, rather they probably come from a common root. Most of the early known usages of the English word come from Scotland, leading some scholars to believe that the word comes from Scandinavian sources. Others disagree, believing that the number of northern citations reflects that the taboo was weaker in Scotland and the north, resulting in more surviving usages. The fact that there are citations, albeit fewer of them, from southern England dating from the same period seems to bear out this latter theory.
There is also an elaborate explanation that has been circulating on the internet for some years regarding English archers, the Battle of Agincourt, and the phrase Pluck Yew! This explanation is a modern jest--a play on words. However, there may be a bit of truth to it. The British (it's virtually unknown in America) gesture of displaying the index and middle fingers with the back of the hand outwards (a reverse peace sign)--meaning the same as displaying the middle finger alone--may derive from the French practice of cutting the fingers off captured English archers. Archers would taunt the French on the battlefield with this gesture, showing they were intact and still dangerous. The pluck yew part is fancifully absurd. This is not the origin of the middle finger gesture, which is truly ancient, being referred to in classical Greek and Roman texts.
