The fatras is defined generally as an irrational or obscure piece of verse
, in a form which originated in the Middle Ages. Also called fatrasie
Paraphrasing Isadore Silver, author of the entry on "fatras" that appears in the Princeton Encyclopedia of Poetry and Poetics (Enlarged ed. 1974), the style is typically lively and joyous, full of wordplay, ridiculous associations, and intentional nonsense.
Two primary subtypes are usually recognized: the fatras possible offers a coherent text, while the fatras impossible seeks to make almost no sense at all, similar to a later form, the coq-à-l'âne.
It is not the incoherence of the content, though, which constitutes a fatras, but its particular form: a strophe of 11 lines, the first and last of which form a distich placed at the beginning as the theme of the composition. This defines the fatras simple.
A fatras double is formed from this by "restating the initial [distich] in reverse order, and adding a second strophe of ten lines ending with an eleventh, a restatement of line one of the [distich]." (Source: E. Langlois, Recueil d'Arts de seconde rhétorique (1902; p.192, n. 1))
An example of an English-language fatras simple
is linked here