rouille

Latin "rubigo" was rust, the reddish iron oxide that corroded metal and blighted grain crops. Old French inherited the word as "roille" and later "rouille," which by the medieval period named both the oxidation on iron and the color associated with it. The leap to a sauce is purely visual: rouille looks like rust.

The sauce itself is a Marseille invention, made from red chili peppers, garlic, olive oil, and sometimes saffron or bread crumbs as a thickener. It arrived at the table as an accompaniment to bouillabaisse, the Marseille fish stew that fishermen made from unsold catch, and was spread on slices of toasted bread set in the bowl. Without rouille, bouillabaisse is simply fish soup.

The heat in rouille was historically provided by piment, a generic term for any capsicum that reached southern France from the Americas via Spain in the sixteenth century. Before capsicum arrived, versions of the sauce used black pepper and dried herbs. The modern form of rouille stabilized by the nineteenth century, when Marseille's fishing ports were at their commercial peak.

Food writers outside France began using "rouille" untranslated in the twentieth century, acknowledging that "rust sauce" or "red pepper mayonnaise" captures nothing of the original. Julia Child described the sauce in "Mastering the Art of French Cooking" of 1961, spelling out the technique that prevents the garlic emulsion from breaking. The word now appears on menus in any city where bouillabaisse is taken seriously.