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1.
Something that one knows they don't know.
There are known unknowns, that is to say that there are things that we know we don't know. (whiskey tango foxtrot, Donald?)
by Bushmonkey October 01, 2006
 
2.
These are the things that you know you don't know. Coined by Donlad Rumsfeld, former US Secretary of Defense.
Boss: Are you saying we don't know what this project will cost?

You: Yep, it's a known unknown!
by seven8521 October 31, 2009