The Spanish word 'viejo' meaning 'old' comes from the Vulgar Latin 'veclus', which developed from Classical Latin 'vetulus' meaning 'somewhat old'. The Latin 'vetulus' was formed by combining the root 'vetus' (meaning 'old') with the diminutive suffix '-ulus'. Over time, through regular sound changes, 'vetulus' evolved to 'veclus' in Vulgar Latin, and finally to 'viejo' in Spanish.

Some related Spanish words include 'veterano' (veteran) and 'veterinario' (veterinarian), which both come from the same Latin root 'vetus' meaning 'old'. A veterinarian was originally someone who took care of old or experienced working animals, while a veteran is someone with long-standing experience.

English speakers might recognize the connection to words like 'veteran' and 'veterinary', which share the same Latin ancestor 'vetus'. The word 'inveterate' (meaning deeply established or long-standing) also comes from this root, literally meaning 'made old' in Latin. These connections help remind us that 'viejo' is about age and long-standing existence.