mentonnière

beaver (lower part of a helmet); chinrest

“« Et je dis aussi que bien que ce soit un armet, ce n’est pas un armet entier. ¶ – Non certes, s’écria don Quichotte, car il lui manque une moitié, qui est la mentonnière. »” — "And I say that as much as it is a helmet, it is not an entire helmet" ¶ "Certainly not," exclaimed Don Quixote, "as it is missing a half, which is the beaver."

From menton (“chin”) + -ière.