The Spanish verb 'custodiar' (meaning 'to guard' or 'to protect') comes from the Latin word 'custos' meaning 'guardian.' From 'custos,' Latin developed the noun 'custodia' meaning 'guardianship' or 'protection.' This word entered Spanish as 'custodia' (meaning 'custody'), and then the verb 'custodiar' was formed by adding the Spanish infinitive suffix '-ar' to create a verb meaning 'to guard' or 'to protect.'

A simpler and more common related Spanish word is 'custodia,' which means 'custody' or 'safekeeping.' While 'custodia' refers to the state or responsibility of protecting something, 'custodiar' is the action of actually guarding or protecting it.

English speakers might recognize the connection to words like 'custody,' 'custodian,' and 'custodial,' which all come from the same Latin root 'custos.' A custodian is literally a guardian or keeper, while custody refers to the protective care or guardianship of something or someone - meanings that closely align with the Spanish 'custodiar.'