late 14c., "prayer-book, layperson's devotional manual," also "school book" (senses not distinguished in Middle English, as reading was taught from prayer books), from Medieval Latin primarium, from Latin primus "first" (see prime (adj.)), on the notion of "a first book." The word also might be all or in part from prime (n.) in the time sense on the same notion as a book of hours. Meaning "small introductory book on any topic" is from 1807.
updated on November 08, 2020