Flen flyys and freris

This is a post especially for Michael and Florian who are still looking for the solution of the January PBC. It was related to the origin of the "f" word, mentioned the first time in a poem called "Flen, flyys, and freris" (= "Fleas, flies, and friars").

Explanation on wikipedia:

The usually accepted first known occurrence is in code in a poem in a mixture of Latin and English composed some time before 1500. 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". Removing the substitution cipher on the phrase "gxddbov xxkxzt pg ifmk" yields "non sunt in coeli, quia fvccant vvivys of heli", which translated means "they are not in heaven because they fuck wives of Ely" (fvccant is a fake Latin form). The phrase was coded likely because it accused some Church personnel of misbehaving; it is uncertain to what extent the word "fuck" was considered acceptable at the time.

I thought this would be easier because when you google for "fleas AND flies AND friars" the wikipedia article is the first search result. Followed by a lot more links to the poem and explanations.