Where does the word "fuck" come from?
The Oxford English Dictionary says the origin is unknown, although it may be derived from a Middle English word "fuken".
The Random House Historical Dictionary of American Slang claims that it is an
English reflex of a widespread Germanic form; cf. Middle Dutch fokken 'to thrust, copulate with'; Norwegian dialect fukka 'to copulate'; Swedish dialect focka 'to strike, push, copulate' and fock 'penis'
The Cassel Dictionary of Slang agrees with the OED but does note the Random House claim.
The alt.usage.english FAQ (http://alt-usage-english.org/big_faq.html#fxfuckxx) cites the Random House etymology, but also notes claims for deriviations from the Indo-European peig 'hostile' or peuk 'to prick'.
The Snopes urban legend site (http://www.snopes.com/language/acronyms/fuck.htm) refers to the alt.usage.english FAQ to debunk the claim that fuck is an acronym of some kind.
What it all boils down to is ... nobody knows where the fuck "fuck" comes from.
This, by the way, is a typical question that we get from the participants in the Our Whole Lives program. At the end of every session, each of the youth is given a blank index card and a pencil. They are each required to write something on the card - either some question that they have, or something along the lines of "no question" - and put the card in a Question Box. We facilitators divvy up the cards that have actual questions, research the answers, and read the questions and answers aloud to the entire group the following meeting. The idea is to provide a completely anonymous channel for these youth (twelve through fourteen years old) to get answers to questions about sex without the embarrassment of having anybody know what they asked.