Written form only attested from early 16c. OED 2nd edition cites 1503, in the form fukkit; earliest appearance of current spelling is 1535 -- "Bischops ... may fuck thair fill and be vnmaryit" [Sir David Lyndesay, "Ane Satyre of the Thrie Estaits"], but presumably it is a much more ancient word than that, simply one that wasn't likely to be written in the kind of texts that have survived from O.E. and M.E. Buck cites proper name John le Fucker from 1278. The word apparently is hinted at in a scurrilous 15c. poem, titled "Flen flyys," written in bastard L. and M.E. The relevant line reads:
Non sunt in celi
quia fuccant uuiuys of heli
"They [the monks] are not in heaven because they fuck the wives of Ely." Fuccant is pseudo-L., and in the original it is written in cipher. The earliest examples of the word otherwise are from Scottish, which suggests a Scandinavian origin, perhaps from a word akin to Norw. dial. fukka "copulate," or Swedish dial. focka "copulate, strike, push," and fock "penis." Another theory traces it to M.E. fkye, fike "move restlessly, fidget," which also meant "dally, flirt," and probably is from a general North Sea Gmc. word, cf. M.Du. fokken, Ger. ficken "fuck," earlier "make quick movements to and fro, flick," still earlier "itch, scratch;" the vulgar sense attested from 16c.