ok robin, this one was kinda tricky, but here's what i did.
1) first, go to www.m-w.com
2) type in "eminence"
3) in the little box with the words to choose from, click on "eminence grise"
4) read entry.

here's the def: a confidential agent; especially : one exercising unsuspected or unofficial power

One look is a nice tool and is generally better about finding variations.

Hmm... Looks like Google isn't bilingual enough to suggest the spelling "grise"...

As for origin, I'd assumed the "grise" of the power-behind-the-throne referred to gray hair (age and wisdom)... but the picture above really illustrates the idea of a gray-robed monk being the power behind a scarlet-robed cardinal.

http://en.wikipedia.org/wiki/Power_behind_the_throne

" inence grise (French: "gray eminence"), a powerful advisor or decision-maker who operates secretly or otherwise unofficially. This phrase originally referred to Cardinal de Richelieu's right hand man, François Joseph du Tremblay, a Capuchin priest who wore gray robes."

Only problem with this is that today's Capuchin's wear brown -- and their brown robes are said to be behind the word cappuccino...
http://www.billcasselman.com/wording_room/cappuccino.htm

Duh... after getting an html error on my first draft comment, I clipped out a couple of web links, including the one to "the picture" I mentioned... one titled
"L''Eminence Grise"
by Jean-Léon Gérôme

"eminence gris" is a reference to "the elephant in the room"... the looming subject that is never discussed.