The Spanish word 'bazo' (meaning 'spleen') comes from the Latin word 'badius', which meant 'reddish-brown' or 'chestnut-colored'. This etymology reflects the characteristic dark reddish-brown color of the spleen organ. The semantic shift from a color term to an anatomical term occurred as the word evolved from Latin to Spanish, likely because the spleen's distinctive color made it easily identifiable and nameable.

While 'bazo' itself is a relatively simple Spanish word, there aren't many common Spanish words that share its Latin root 'badius'. This makes it somewhat unique in its etymological isolation in modern Spanish vocabulary.

The Latin word 'badius' has also influenced English, though not as directly. The English word 'bay' when referring to the reddish-brown color of horses comes from the same Latin root 'badius'. This connection helps English speakers remember that 'bazo' has a historical connection to a reddish-brown color, even though it now refers specifically to the spleen.