primer (n.3) Look up primer at Dictionary.com
"first layer of dye or paint," 1680s, from prime (v.).
primer (n.1) Look up primer at Dictionary.com
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.
primer (n.2) Look up primer at Dictionary.com
"explosive cap," 1819, agent noun from prime (v.).