Meaning

noun an introductory textbook

noun any igniter that is used to initiate the burning of a propellant

noun the first or preliminary coat of paint or size applied to a surface

originFrom Middle English primer, primere, from Medieval Latin primarius and primarium (“prayer book”) possibly via Anglo-Norman primer (“prayer book”), from prima (“prime the liturgical hour and office”) + -arius and -arium (suffix forming related objects). Its use for schoolbooks derived from the late medieval and early modern use of such prayer books to teach reading.

Etymology from Wiktionary, CC BY-SA 3.0