We borrowed "réchauffé" in the early 19th century from the French; it is the past participle of their verb "réchauffer," which means "to reheat." Nineteenth-century French speakers were using it figuratively to designate something that was already old hat-you might say, "warmed over." English speakers adopted that same meaning, which is still our most common. But within decades someone had apparently decided that leftovers would seem more appealing with a French name. The notion caught on. A recipe for "Réchauffé of Beef a la Jardiniere," for example, instructs the cook to reheat "yesterday's piece of meat" in a little water with some tomatoes added, and serve it on a platter with peas and carrots and potatoes. "Réchauffé" shares its root with another English word, "chafing dish," the name of a receptacle for keeping food warm at the table.
French, from réchauffé warmed-over, from past participle of réchauffer to warm over, from ré- re- + chauffer to warm, from Middle French chaufer — more at chafe
