aller au diable vauvert (corrupted into au vert)

The Carthusians having been given a large building at Gentilly by St. Louis, coveted the abandoned mansion of Vauvert (= vallon vert), which they could see from their windows. But to ask for it without a valid reason was to court refusal. So they caused it to be haunted by evil spirits, and the king was soon glad to get rid of this uncanny possession. It is needless to add that the spirits were exorcised directly the monks took possession. It stood in the rue de Vauvert, beyond the Luxembourg, which was until lately called the rue d’Enfer. As this was then a remote suburb of Paris, the expression was equivalent to going to the end of the town, and thus, very far off.