'Gruef' means ditch or moat.
The rue du Fossé, 'um Gruef' in Luxembourgish, got its name because it was built over the ditch that ran in front of the city's second wall. During the German occupation (1940–1944) it was called 'Auf dem Graben', the same name in German. In 1933 it was widened to bring the old town up to the comfort and hygiene of the new 19th-century city.