The Spanish word 'tobillo' (meaning 'ankle') comes from the Latin word 'tubellum', which meant 'small protuberance' or 'small swelling'. This Latin word was itself formed by combining 'tuber' (meaning 'swelling' or 'protuberance') with the diminutive suffix '-ellum'. The ankle was thus named for being a small bump or protrusion on the leg.

While 'tobillo' itself is a fairly basic Spanish word, there are some related Spanish words that come from the same Latin root 'tuber'. For example, 'tubérculo' (tubercle, tuber) and 'tuberculosis' are cognate words, all referring to the concept of swelling or protrusion.

English speakers might recognize the connection to several words derived from the same Latin root 'tuber', such as 'tuber' (like a potato), 'protuberance' (a swelling or bulge), and 'tubercle' (a small rounded projection). The medical term 'tuberculosis' also comes from this root, as it was named for the characteristic small rounded swellings (tubercles) that form in the lungs.