The Spanish word 'soltero', meaning 'unmarried person' or 'single person', comes from the Latin word 'solitarius', meaning 'solitary' or 'isolated'. The word evolved through Medieval Latin 'soltariu' and Old Spanish 'soltairo' before reaching its modern Spanish form. This etymology reflects how being unmarried was historically associated with living alone or being solitary.

A related Spanish word is 'solo', meaning 'alone' or 'single', which comes from the same Latin root. Another related word is 'soledad', meaning 'loneliness' or 'solitude'. These words share the common theme of being alone or separate from others.

English speakers might recognize the connection to words like 'solitary', 'solitude', and 'solo', which all come from the same Latin root 'solitarius'. The meaning has remained quite consistent across languages, always relating to being alone or separate from others, though the Spanish 'soltero' specifically came to mean an unmarried person.