primer

Primer comes from Middle English primer/primere, from Medieval Latin primarius/primarium meaning 'prayer book,' possibly passed through Anglo-Norman primer, itself rooted in prima, the liturgical hour of prime.

Prayer books like these were used in the late medieval and early modern period to teach people to read, and that practical use gradually gave the word its modern sense of a basic instructional book.

The word is tied to prime, the liturgical hour that named these prayer books in the first place.