The Spanish word 'primogénito' meaning 'firstborn' comes from the Latin word 'primogenitus', which had the same meaning. The Latin word was formed by combining two elements: 'primus' meaning 'first' and 'genitus' meaning 'born' or 'begotten'. This compound literally translates to 'first-born' and has maintained this meaning as it evolved from Latin into Spanish.

Several common Spanish words share roots with 'primogénito': 'primero' (first), which comes from the same Latin 'primus'; and 'género' (gender, type) and 'gente' (people), which are related to the 'genitus' element through the Latin root meaning birth or origin.

English speakers might recognize parts of 'primogénito' in words like 'prime' and 'primary' (from Latin 'primus'), as well as 'generate', 'genesis', and 'genetic' (related to Latin 'genitus' through the concept of creation or birth). The word 'primogeniture' is directly related, referring to the historical right of succession belonging to the firstborn child.