Nappé (pronounced “nappay”) is a French word meaning “to coat.” It’s often used in restaurant kitchens (particularly those run by classically French trained chefs) to refer to a state of liquid thickness (usually sauces). It can also refer to coating food with that thickened liquid. Many home cooks probably haven’t encountered this term, as cookbooks instead tell them to reduce a liquid down until it “coats the back of a spoon,” which means essentially the same thing.