The Spanish word 'casto', meaning 'chaste' or 'pure', comes directly from the Latin word 'castus', which meant 'pure' or 'virtuous'. This is a straightforward inheritance where both the form and meaning have remained remarkably stable from Latin to Spanish, with only minor phonological adjustments typical of Spanish evolution from Latin words.

Some related Spanish words include 'castidad' (chastity), and 'castigar' (to punish) - which originally carried the sense of 'purifying' or 'making pure through correction'. The adjective 'casto' is also commonly seen in the feminine form 'casta' when describing feminine nouns.

English speakers might recognize this word's connection to 'chaste' and 'chastity', which come from the same Latin root 'castus'. The English word 'castigate' (to punish or criticize severely) is also related, sharing the original Latin concept of purification or correction. These cognates make it easier to remember that 'casto' relates to purity and moral virtue.