unknown unknown

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Popularized by Donald Rumsfeld in 2002, though used much earlier.


unknown unknown (plural unknown unknowns)

  1. An uncertainty of unknown magnitude, consequence, structure, and probability characteristics, possibly with completely unsuspected existence; something that we don't know that we don't know.
    • 1970, United States Congress. Senate. Committee on the Judiciary. Subcommittee on Antitrust and Monopoly, Competition in Defense Procurement – 1969: Hearing, Ninety-first Congress, page 166:
      This means there are technical unknowns, unknown unknowns, and technical uncertainties in the contract.
    • 2014 January 29, Kenneth Chang, “Beings not made for space: After years of missions, health problems of astronauts elude doctors”, in International New York Times[1], page 7:
      NASA officials often talk about the "unknown unknowns" – the unforeseen problems that catch them by surprise.


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