Fuck \FUHK\ , verb;
1. To have sexual intercourse with
2. Slang: To treat unfairly or harshly
1. Slang: (used to express anger, disgust, or peremptory rejection)
1. An act of sexual intercourse
2. A partner in sexual intercourse
3. Slang: A person, especially one who is annoying or contemptible
Fuck is one elusive word, etymologically speaking. It's likely been taboo for it's whole life, so it is extremely difficult (probably impossible) to say how long that life has been. It is first attested in the early 16th century, but the existence of a surname Fucker (attested in 1278) and a possible *wink*wink* at it in a 15th century poem suggests it older. There are several theories as to the origin of the word, some of which are as plausible as they are unprovable and some of which are just plain wrong.
That 15th century poem hints at fuck with fuccant, which is pseudo-Latin. This probably has nothing to do with the etymology, though, because all other attestations and possibly related words are decidedly Germanic.
The oldest examples of the word are from Scottish, which suggests Scandinaivian roots. There is a Norwegian dialectal word fukka ("copulate") and two Swedish dialectal words focka ("copulate, strike, push") and fock ("penis") that may be related to our English naughty word.
Some theorize that the word evolved out of Middle English fyke which meant "move restlessly, fidget" and "dally, flirt." That word is probably from an North Sea Germanic source that also spawned Middle Dutch fokken and German ficken. Originally German ficken meant "itch, scratch," then "make quick movements to and fro, flick," then "fuck."
The Middle English slang word for "have sexual intercourse" was swive and there is a suspiciously similar word firk ("to press hard, beat") which are unrelated to fuck. Same with French foutre and Italian fottere.
There are also various internet etymologies that are just ridiculous. Fuck is not an acronym for "for unlawful carnal knowlegde" and pluck yew has nothing to do with it.