means A light, airy dish—sweet like chocolate or savory like salmon—whipped to a soft, foamy texture, or a similarly fluffy product for styling hair.

from Straight from French mousse, meaning 'foam' or 'froth,' which traces back to Late Latin mulsa, 'mead' or 'honeyed drink'—though it tangled along the way with a Germanic word for moss (think of the soft, spongy stuff underfoot). The culinary sense bubbled up in the 19th century; the hair-styling sense is a 20th-century borrowing of the same foamy idea.