"closed book" meaning in English

{
  "antonyms": [
    {
      "word": "open book"
    }
  ],
  "forms": [
    {
      "form": "closed books",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "closed book (plural closed books)",
      "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": "1906, Mary Cholmondeley, Prisoners",
          "text": "Her courteous, sedate, inexorable husband, whose will she could not bend, whom she could not cajole, whose mind was a closed book to her — a book which had lain by her hand for three years, which she had never had the curiosity to open!",
          "type": "quotation"
        },
        {
          "ref": "1962 April, R. K. Evans, “The Acceptance Testing of Diesel Locomotives”, in Modern Railways, page 266",
          "text": "Like the activities of British Railways Research Department, developing, testing and modifying diesel locomotives before their acceptance for service is another behind-the-scenes activity which is a closed book to most passengers and to many railwaymen.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling."
      ],
      "id": "en-closed_book-en-noun-aYGTSwr8",
      "links": [
        [
          "understood",
          "understood"
        ],
        [
          "incomprehensible",
          "incomprehensible"
        ],
        [
          "puzzling",
          "puzzling"
        ]
      ],
      "raw_glosses": [
        "(idiomatic) A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling."
      ],
      "related": [
        {
          "word": "enigma"
        },
        {
          "word": "mystery"
        },
        {
          "word": "secret"
        }
      ],
      "tags": [
        "idiomatic"
      ]
    }
  ],
  "sounds": [
    {
      "audio": "en-au-closed book.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/7/77/En-au-closed_book.ogg/En-au-closed_book.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/7/77/En-au-closed_book.ogg",
      "tags": [
        "Australia"
      ],
      "text": "Audio (AU)"
    }
  ],
  "word": "closed book"
}

{
  "antonyms": [
    {
      "word": "open book"
    }
  ],
  "forms": [
    {
      "form": "closed books",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "closed book (plural closed books)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "enigma"
    },
    {
      "word": "mystery"
    },
    {
      "word": "secret"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English idioms",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with audio links",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "1906, Mary Cholmondeley, Prisoners",
          "text": "Her courteous, sedate, inexorable husband, whose will she could not bend, whom she could not cajole, whose mind was a closed book to her — a book which had lain by her hand for three years, which she had never had the curiosity to open!",
          "type": "quotation"
        },
        {
          "ref": "1962 April, R. K. Evans, “The Acceptance Testing of Diesel Locomotives”, in Modern Railways, page 266",
          "text": "Like the activities of British Railways Research Department, developing, testing and modifying diesel locomotives before their acceptance for service is another behind-the-scenes activity which is a closed book to most passengers and to many railwaymen.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling."
      ],
      "links": [
        [
          "understood",
          "understood"
        ],
        [
          "incomprehensible",
          "incomprehensible"
        ],
        [
          "puzzling",
          "puzzling"
        ]
      ],
      "raw_glosses": [
        "(idiomatic) A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling."
      ],
      "tags": [
        "idiomatic"
      ]
    }
  ],
  "sounds": [
    {
      "audio": "en-au-closed book.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/7/77/En-au-closed_book.ogg/En-au-closed_book.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/7/77/En-au-closed_book.ogg",
      "tags": [
        "Australia"
      ],
      "text": "Audio (AU)"
    }
  ],
  "word": "closed book"
}