open-book

The compound "open-book" is a transparent metaphor from Modern English. The adjective "open," with its ancient Germanic roots conveying accessibility and lack of obstruction, is combined with "book," an object symbolizing contained knowledge. Literally describing a book that is not closed, the term evolved in the late 19th and early 20th centuries to figuratively denote situations where knowledge sources are accessible, most notably in educational testing. The logic is direct: just as an open book reveals its contents, an "open-book" exam allows access to information, and by further extension, a person described as an "open book" makes their inner thoughts readily apparent.

Structure: open (not closed or secret) + book (a set of written or printed pages) - open: From Old English 'openian' (to open), of Germanic origin; related to Old Norse 'opinn'. Functions as an adjective meaning not closed, accessible, or unconcealed. - book: From Old English 'bōc' (a written document, book), of Germanic origin; related to Dutch 'boek' and German 'Buch'. Functions as a noun referring to a bound volume of pages or a record.