closed book

See also: closed-book

English

closed book (plural closed books)

(idiomatic) A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling.
- 1906, Mary Cholmondeley, Prisoners:
  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!
- 1962 April, R. K. Evans, “The Acceptance Testing of Diesel Locomotives”, in Modern Railways, page 266:
  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.