Flen flyys is a poem, written some time before 1500, that is chiefly famous for containing the first known written usage in English of the vulgar verb "fuck". In fact the usage was "fuccant", a hybrid of an English root with a Latin conjugation, and was disguised in the text by a simple code, in which each letter was replaced with the next letter in the alphabet.

Written half in English and half in Latin, the poem satirised Carmelite monks in the English county of Cambridgeshire. The poem takes its name from the opening line Flen, flyys and freris meaning "fleas, flies and friars".

The famous line reads "Non sunt in coeli, quia gxddbov xxkxzt pg ifmk." meaning "They [the friars] are not in heaven, since ..." followed by words that when decoded read "fvccant vvivys of heli", a Latin/English mix that means "...they fuck the wives of Ely" [a town].