Merriam-Webster Collegiate Dictionary - eminence grise

 noun  (plural eminences grises)  Etymology: French, literally, gray eminence, nickname of Pere Joseph (Francois du Tremblay) died 1638 French monk and diplomat, confidant of Cardinal Richelieu who was known as Eminence Rouge red eminence; from the colors of their respective habits  Date: 1925  1. a confidential agent; especially one exercising unsuspected or unofficial power  2. a respected authority; specifically elder statesman the ~ of classical music