"unknown unknown" meaning in English

{
  "etymology_text": "Popularized by Donald Rumsfeld in 2002, though used much earlier.",
  "forms": [
    {
      "form": "unknown unknowns",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "unknown unknown (plural unknown unknowns)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "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",
          "text": "This means there are technical unknowns, unknown unknowns, and technical uncertainties in the contract.",
          "type": "quotation"
        },
        {
          "ref": "2002 February 12, Donald Rumsfeld, “Defense.gov News Transcript: DoD News Briefing – Secretary Rumsfeld and Gen. Myers, United States Department of Defense (defense.gov)”, in Defense.gov Transcript Archive, archived from the original on 2016-08-10",
          "text": "Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.",
          "type": "quotation"
        },
        {
          "ref": "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, page 7",
          "text": "NASA officials often talk about the \"unknown unknowns\" – the unforeseen problems that catch them by surprise.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "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."
      ],
      "id": "en-unknown_unknown-en-noun-AMRcVtNl",
      "links": [
        [
          "uncertainty",
          "uncertainty"
        ]
      ],
      "related": [
        {
          "word": "known known"
        },
        {
          "word": "known unknown"
        },
        {
          "word": "unknown known"
        },
        {
          "word": "a little knowledge is a dangerous thing"
        }
      ],
      "synonyms": [
        {
          "word": "unk-unk"
        }
      ],
      "wikipedia": [
        "Donald Rumsfeld",
        "There are known knowns"
      ]
    }
  ],
  "word": "unknown unknown"
}

{
  "etymology_text": "Popularized by Donald Rumsfeld in 2002, though used much earlier.",
  "forms": [
    {
      "form": "unknown unknowns",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "unknown unknown (plural unknown unknowns)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "known known"
    },
    {
      "word": "known unknown"
    },
    {
      "word": "unknown known"
    },
    {
      "word": "a little knowledge is a dangerous thing"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "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",
          "text": "This means there are technical unknowns, unknown unknowns, and technical uncertainties in the contract.",
          "type": "quotation"
        },
        {
          "ref": "2002 February 12, Donald Rumsfeld, “Defense.gov News Transcript: DoD News Briefing – Secretary Rumsfeld and Gen. Myers, United States Department of Defense (defense.gov)”, in Defense.gov Transcript Archive, archived from the original on 2016-08-10",
          "text": "Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.",
          "type": "quotation"
        },
        {
          "ref": "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, page 7",
          "text": "NASA officials often talk about the \"unknown unknowns\" – the unforeseen problems that catch them by surprise.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "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."
      ],
      "links": [
        [
          "uncertainty",
          "uncertainty"
        ]
      ],
      "wikipedia": [
        "Donald Rumsfeld",
        "There are known knowns"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "unk-unk"
    }
  ],
  "word": "unknown unknown"
}