One of the mysteries of the English language finally explained.
1A book containing the correct answers to a test, examination, quiz, etc., used as a guide for marking or checking answers.
2A book answering questions about, or giving information on, a particular topic.
3A book or booklet in which students write the answers to a test, examination, etc.
Early 19th century (in an earlier sense). From answer + book.
answer book/ˈɑːnsə bʊk//ˈansə bʊk/
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