the.com/primer

means A basic introductory book or substance that prepares the way for what comes next — whether that's a child learning to read or a wall learning to hold color.

from Traces back to Latin 'primus,' meaning 'first.' In English, 'primer' first named a basic prayer book for children and beginners — literally a 'first' book of instruction. The paint sense followed the same logic: a primer is the first coat, the preparatory layer, doing the foundational work before the 'real' layer gets the credit.