A short tale in verse, almost invariably in octosyllabic couplets, dealing for the most part from a comic point of view with incidents of ordinary life. The fabliau was an important element in French poetry in the 12th-13th cents, and was imitated by Chaucer, in e.g. 'The Miller's Tale.'
Oxford Companion, you stink. What a stupid, boring definition. Therefore, I give you the definition from The Nokes Companion to Medieval Literature, a highly respected publication I just made up:
A short narrative poem with hilarious dirty jokes. Good fabliaux focus on several key elements: jokes about genitalia, clergy, cuckolds, stupid peasants, viragoes, and scatological jokes (esp. including jokes about farting). The fabliau has suffered as a genre because it cannot generally be taught until the post-secondary level of education, however it has also secured the position of medieval literature as the coolest literary period one can study.
Now, that's a definition!
I'm afraid I can't really tell you the title of this fabliau in English (in order to maintain a generally PG13 rating for the blog), so I'll link to it using the French title: "Le Chevalier Qui Fist Parler les Cons."*
*My apologies to any French parents out there whose children may have stumbled onto this post ... but, hey, at least they're getting some culture!