In many archive documents relating to Fond-de-Gras, there are different spellings: “Fond-de-Grâce”, “Fond de Graas”, “Fonds-de-Gras”; this can make the pronunciation of the place somewhat uncertain.
Only one pronunciation is correct, and that is to say the final “s” correctly.
In fact, the vast area that extends before the descent to Fond-de-Gras is called “Grasskop” in Luxembourgish, which can be translated as “prairie-hill”, referring precisely to the grasslands that cover these heights.
In contrast, the area below the Grasskop is called “Fond-de-Gras”, because the valley is also covered with grass due to the persistent humidity. “Fond-de-Gras’ is therefore a singular toponym, since it combines French and German/Luxembourgish vocabulary.
This explanation seems to be the most plausible and supersedes others, notably the one referring to a certain Monsieur Gras who was active there, or the macabre reference to hypothetical executions that were perpetrated there (Grâce; Gnade).
– Klein René, “Rund um den Fond-de-Gras. Über Herkunft und Aussprache des Namens”, in “Mutations. Mémoires et perspectives du Bassin minier” 8/2015, “Le Fond-de-Gras – Histoire(s) d’un lieu : des origines à nos jours” Luxembourg, 2015.
– Claude Arendt, “Maer Rollenger” website link https://www.maer-rollenger.com/tourismus/fond-de-gras-fdg/