Fuck
 

In ancient England a person could not have sex unless you had consent of the King (unless you were in the Royal Family).

When anyone wanted to have a baby, they got consent of the King, the King gave them a placard that they hung on their door while they were having sex.

The placard had F.U.C.K. (Fornication Under Consent of the King)
on it.

I heard that origin is false.

Pulled from a source:

"The acronymic explanation of the origin of 'fuck' takes one of two paths: Fornication Under Consent of the King or For Unlawful Carnal Knowledge. Dealing with the first of these, though it's pleasing to think couples looking to procreate in those Dark Old Days had to first obtain the sovereign's persmission and then post a notice of what they were up to so all the neighbors could enjoy a good snicker, a moment's thought should set that one to rest. Were the king responsible for handing out such permissions, he wouldn't have time to do anything else (or even to keep up with that one task). Likewise, though there have been times when conquering forces have engaged in rape, it wasn't by royal fiat at the behest of a king looking to further dispirit the conquered.

One last nail in the coffin of the 'fornication under consent of the king' origin comes from the word 'fornication' itself. Though many reasonably conclude fornication is the old-time word for having sex, the term specifically excludes the physical union of man and wife. One can fornicate premaritally or extramaritally, but not intramaritally. In light of this, any claim wedded couples trying to entice the stork down their chimney were granted fornication permits crashes against the rock of the wrong word being used."

fornicate you, you bastard

I don't know why I care, but...

According to the alt.usage.english FAQ:


[Fuck] is a very old word, recorded in English since the 15th century (few acronyms predate the 20th century), with cognates in other Germanic languages. The Random House Historical Dictionary of American Slang (Random House, 1994, ISBN 0-394-54427-7) cites Middle Dutch fokken = "to thrust, copulate with"; Norwegian dialect fukka = "to copulate"; and Swedish dialect focka = "to strike, push, copulate" and fock = "penis". Although German ficken may enter the picture somehow, it is problematic in having e-grade, or umlaut, where all the others have o-grade or zero-grade of the vowel.
AHD1, following Pokorny, derived "feud", "fey", "fickle", "foe", and "fuck" from an Indo-European root peig2 = "hostile"; but AHD2 and AHD3 have dropped this connection for "fuck" and give no pre-Germanic etymon for it. Eric Partridge, in the 7th edition of Dictionary of Slang and Unconventional English (Macmillan, 1970), said that "fuck" "almost certainly" comes from the Indo-European root *peuk- = "to prick" (which is the source of the English words "compunction", "expunge", "impugn", "poignant", "point", "pounce", "pugilist", "punctuate", "puncture", "pungent", and "pygmy"). Robert Claiborne, in The Roots of English: A Reader's Handbook of Word Origin (Times, 1989) agrees that this is "probably" the etymon. Problems with such theories include a distribution that suggests a North-Sea Germanic areal form rather than an inherited one; the murkiness of the phonetic relations; and the fact that no alleged cognate outside Germanic has sexual connotations.

'fuck' comes from 'fichen', German for "to strike."