Fuehrerbefehl, literally "leader command". An order from a superior, boss, commander. In Nazi Germany, an order emanating directly from Hitler himself. Although there were many "fuehrers" at all levels in the Third Reich, there was only one capital-F Fuehrer -- "Der Fuehrer" or "The Leader". Compare the phrase "zu befehl" meaning "at your command," a common reply by soldiers to a direct command by a superior officer.
