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late 14c., "prayer-book," also "school book" (senses not distinguished in Middle Ages, as reading was taught from prayer books), from Medieval Latin primarius, from Latin primus "first" (see prime (adj.)). The word also might be all or in part from prime (n.) on the same notion as a "Book of Hours." Meaning "small introductory book on any topic" is from 1807.
"explosive cap," 1819, agent noun from prime (v.).
"first layer of dye or paint," 1680s, from prime (v.).