I don’t know how I’ve made it into my mid-50s without having seen this story before, but I love it. In the course of Mark Liberman’s Language Log post on “typographical bleeping,” he quotes the American Heritage Dictionary as follows:
The obscenity fuck is a very old word and has been considered shocking from the first, though it is seen in print much more often now than in the past. Its first known occurrence, in code because of its unacceptability, is in a poem composed in a mixture of Latin and English sometime 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,” that is, “fleas, flies, and friars.” The line that contains fuck reads “Non sunt in coeli, quia gxddbov xxkxzt pg ifmk.” The Latin words “Non sunt in coeli, quia,” mean “they [the friars] are not in heaven, since.” The code “gxddbov xxkxzt pg ifmk” is easily broken by simply substituting the preceding letter in the alphabet, keeping in mind differences in the alphabet and in spelling between then and now: i was then used for both i and j; v was used for both u and v; and vv was used for w. This yields “fvccant [a fake Latin form] vvivys of heli.” The whole thus reads in translation: “They are not in heaven because they fuck wives of Ely [a town near Cambridge].”
If only I’d known that in seventh grade! (The OED apparently doesn’t consider this an actual attestation, because their first citation is “a1503 DUNBAR Poems lxxv. 13 Be his feiris he wald haue fukkit”; I don’t know whether that’s because of the code or because it’s fake Latin rather than straight English. And a correspondent cites the lines in Notes and Queries for Oct. 13, 1855 without the encoded words, saying “My omissions are put in cypher by Mr. Wright, and are not producible [sic].” He has misunderstood Wright’s own fastidious words in his Reliquiæ Antiquæ: Scraps from Ancient Manuscripts: “The expressions concealed by the cypher, as in the MS., are rather gross, and do not speak much for the morals of the Carmelites of Cambridge.” At any rate, at the last link you can see the entire poem, such as it is, with the further encoded line “Fratres cum knyvys goth about and txxkxzv nfookt xxzxkt.”)
Oh, and for the Latinists among you, Marie Borroff, in a footnote to her Traditions and Renewals: Chaucer, the Gawain-Poet, & Beyond (p. 209), says “The incorrect form cœli may have been substituted for cœlis for the sake of the rhyme with heli.” I’ll point out also that the h of heli is a purely orthographical flourish, not to be pronounced; the town of Ely derives its name from eels, not heels.