The Spanish word 'justo' meaning 'just' or 'fair' comes from the Latin word 'iustus' with the same meaning. The Latin 'iustus' was formed by combining the root 'ius' meaning 'law, right, justice' with the past participle suffix '-tus'. Over time, as Latin evolved into Spanish, 'iustus' transformed into 'justo', with the initial 'i' becoming 'j' (which has a similar sound in Spanish).

Several common Spanish words share this root, including 'justicia' (justice), 'justificar' (to justify), and 'ajustar' (to adjust or fit). All these words carry the core concept of rightness, fairness, or making things correct that comes from the original Latin root.

English speakers will recognize many cognates from this same Latin root, including 'just', 'justice', 'justify', and 'adjust'. These English words followed a parallel evolution from the same Latin source, which is why they share both similar meanings and spellings with their Spanish counterparts. The English word 'just' is particularly close to the Spanish 'justo', both in meaning and form.