I hear this expression quite often but i don't understand it. I checked in numerous dictionaries but it's not in there.

expression: guy behind the guy behind the guy behind that guy
Sometimes it's extended even more than that. Anyone?

thanks in advance
Certainly I've heard "the guy behind the guy," but not repeated as you have it.

"The guy" is the guy in charge, the big man, the decision maker, etc. "The guy behind the guy" would be the person who helped get "the guy" into power, who helps with his research, his decision making, etc. Sometimes the person who really knows what's going on doesn't have the charisma or the leadership qualities to be "the guy" but he can be the guy behind the guy and still have quite a bit of power.

Once you have more than the one "behind the" the person is making a play of the expression.
In some contexts, it could mean:
éminence grise
Pronunciation: mnsgrz
Function: noun
Inflected Form(s): plural éminences grises \"\

Etymology: French, literally, gray eminence, nickname of Père Joseph (François Le Clerc du Tremblay) died 1638 French monk and diplomat who was confidant of Cardinal Richelieu died 1642 French statesman and cardinal who was styled Éminence Rouge red eminence; from the colors of their respective habits

: a confidential agent; especially : one exercising unsuspected or unofficial power


Someone acting in the shadow, in background.
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You hear this quite often? Where and under what circumstances? I suppose that it is referring to the final authority in some matter; either that, or to a person sandwiched between two mirrors.
 BarbaraPA's reply was promoted to an answer.
Hi guys,

The standard saying is that 'Behind every succesful man is a woman'.

Best wishes, Clive
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 Marius Hancu's reply was promoted to an answer.