The poem, which satirizes the Carmelite friars of Cambridge, England, takes its title, "Flen flyys", from the first words of its opening line, Flen, flyys, and freris ("Fleas, flies, and friars").
The line that contains fuck reads Non sunt in coeli, quia gxddbov xxkxzt pg ifmk.
These roots, even if cognates, are not the original Indo-European word for to copulate, but Wayland Young argues that they derive from the Indo-European *bug– ("be", "become"), or as causative "create" [see Young, 1964].
A possible intermediate might be a Latin 4th-declension verbal noun *fūtus, with possible meanings including "act of (pro)creating".
There are many common phrases that employ the word, as well as compounds that incorporate it, such as motherfucker, fuckwit and fucknut.