The Spanish word 'primer' comes from 'primero', which is a shortened form used before masculine nouns. Both words trace back to the Latin word 'primarius' meaning 'of the first rank'. This Latin word was formed by combining 'primus' meaning 'first' with the suffix '-arius' meaning 'belonging to'. Over time, this evolved into Spanish 'primero', and then the shortened form 'primer' used before nouns.

The most obvious related Spanish word is 'primero' (first), which is the full form of 'primer'. Other common related Spanish words include 'primo' (cousin, literally 'first relation'), 'primavera' (spring, literally 'first season'), and 'primitivo' (primitive, original).

Several English words share the same Latin root 'primus'. These include 'primary', 'prime', 'primitive', and 'principal'. Just as 'primer' in Spanish refers to being first, these English words all carry connotations of being first or most important. The English word 'primer' (an elementary textbook) is also related, as it was historically the 'first' book students would use to learn reading.