The Spanish word 'duodécimo' (meaning 'twelfth') comes from the Latin word 'duodecimus', which was formed by combining two Latin number words: 'duo' meaning 'two' and 'decem' meaning 'ten'. This combination literally represented 'two and ten' or 'twelve', with the appropriate Latin suffix to make it an ordinal number (meaning 'twelfth' rather than just 'twelve').

The word maintained its meaning and underwent minimal changes as it evolved from Latin to Spanish, mainly adapting to Spanish spelling conventions with the addition of the accent mark over the 'e'.

Some simpler related Spanish words include 'dos' (two) and 'diez' (ten), which come from the same Latin roots 'duo' and 'decem' respectively. Another common related word is 'docena' (dozen), which is conceptually related as it refers to a group of twelve items.

Several English words share these Latin roots. The prefix 'duo-' meaning 'two' appears in words like 'duet' and 'duplex'. The root relating to 'ten' appears in 'decimal' and 'decade'. The word 'duodecimal' is particularly close to 'duodécimo', as it also means 'relating to twelve' or 'based on twelve'. Even the word 'dozen' is distantly related, coming from the same concept of 'two plus ten'.