Flen flyys (Middle English: "Fleas (and) flies") is the colloquial name and first words of an anonymous, untitled poem, written about 1475 or earlier, famous for containing an early 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 of the time (so that fuccant is written as gxddbou).
Written half in English and half in Latin, the poem satirised Carmelite friars in the English county of Cambridgeshire. The poem takes its name from the opening line Flen, flyys and freris meaning "fleas, flies and friars".