The Spanish word 'moco' comes directly from the Latin word 'muccus', meaning 'mucus' or 'slime'. The word underwent a simple phonetic change as Latin evolved into Spanish, with the double 'cc' simplifying to 'c' and the final 'us' becoming 'o', which is a common pattern in Spanish words derived from Latin.

Some related Spanish words include 'mocoso' (snotty or runny-nosed), and 'mocar' (to blow one's nose). These words are all built from the same root 'moco' and relate to nasal mucus.

English speakers might recognize the connection to the word 'mucus', which comes from the same Latin root 'muccus'. The medical term 'mucous' and words like 'mucosity' are also related. While English preserved more of the original Latin spelling, Spanish simplified it to 'moco', but both languages maintained the original meaning related to nasal secretions.