- (idiom) A person or thing that cannot be easily understood; someone or something uncomprehensible or puzzling.
- "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!" — w:Prisoners, Prisoners by Mary Cholmondeley
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