The Spanish word 'casual' comes from the Latin word 'casualis' meaning 'by chance'. This Latin word was formed by combining 'casus' (meaning 'chance' or 'accident') with the suffix '-alis' (meaning 'relating to'). Going back further, 'casus' derived from the Latin verb 'cadere' meaning 'to fall' or 'to happen', showing how the concept of chance was originally connected to the idea of things that fall or happen unexpectedly.

Some related Spanish words include 'caso' (case, instance), which comes directly from Latin 'casus', and 'caer' (to fall), which comes from Latin 'cadere'. The connection between these words helps show how the meaning evolved from physical falling to chance occurrences to casual events.

English speakers might recognize the connection to several familiar words: 'casual' itself, which has the same Latin origin and similar meaning; 'case' (from Latin 'casus'); and 'cascade' (from Latin 'cadere', to fall). The word 'chance' is also distantly related, coming through Old French from Latin 'cadentia', another derivative of 'cadere'.