>what says the OED etymology?

in the interests of effing completeness, here it is:

[Early mod.E fuck, fuk, answering to a ME. type *fuken (wk. vb.) not found; ulterior etym. unknown. Synonymous G. ficken cannot be shown to be related.]
For centuries, and still by the great majority, regarded as a taboo-word; until recent times not often recorded in print but frequent in coarse speech.
a1503 Dunbar Poems lxxv. 13 Be his feiris he wald haue fukkit. 1535 Lyndesay Satyre 1363 Bischops+may fuck thair fill and be vnmaryit....[etc]