Latin for "first, the first;" see prime (adj.). In various phrases, e.g. primus inter pares "first among equals."
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.
"first coat of paint or other material given to any surface," c. 1600, verbal noun from prime (v.). Meaning "act of priming a firearm" is by 1590s; that of "gunpowder in the pan of a firearm" is from 1620s.