éminence grise
noun
[ ˌɛmɪnɒ̃s ˈɡriːz ]
• a person who exercises power or influence in a certain sphere without holding an official position.
• "for years he has been an éminence grise operating in the shadows of party policy-making"
Origin:
1930s: French, literally ‘grey eminence’. The term was originally applied to Cardinal Richelieu's grey-cloaked private secretary, Père Joseph (1577–1638).
