Carl Von Clausewitz is the author of that term. It is part of his discussion on Friction of War in Book One of his 8 books, On War.
When Hillary's ambassador was attacked, raped, and slaughtered, it was in a single attack that was predicted days in advance. The country may be in a pseudo-state of war, but not so much that the ambassador required any security.
In other words, if her claim of "Fog of War" is to be believed, then she must answer the question: "Why did you have an ambassador in a war zone without any security?"
She is misusing the phrase. She must take the spotlight off of the President. Also, she is obviously attempting to distract from her incompetence and the glaring lack of a resignation. This "Fog of War" line is just meant to buy another day or two.
Hillary should consult her staff about terms such as "Fog of War." Many career State Department employees have read Clausewitz and could suggest a better
Wikipedia - On War
On War - Table of Contents
On War - Amazon