[This is a fast-access FAQ excerpt.]
"Fuck" does NOT stand for "for unlawful carnal knowledge" or
"fornication under consent of the king". It is not an acronym for
anything at all.
It 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.