The Spanish word 'celos' (meaning 'jealousy') has a fascinating journey that starts with the Ancient Greek word 'ζεῖν' (zein), which meant 'to boil'. From this root emerged the Greek word 'ζῆλος' (zelos), which developed to mean both 'zeal' and 'jealousy' - perhaps reflecting how both emotions can 'boil up' inside a person. This word was borrowed into Latin as 'zelus', keeping similar meanings of 'zeal' and 'jealousy'. The Latin term evolved into Spanish 'celo', and the plural form 'celos' became the common way to express the concept of jealousy in Spanish.

A related Spanish word is 'celoso/celosa', meaning 'jealous'. You might also encounter 'celar', which means 'to be jealous' or 'to watch over jealously'. These words all share the same root and form a family of terms relating to jealousy and zealous protection.

English speakers might recognize this etymology in words like 'zeal', 'zealous', and 'zealot', which come from the same Greek root. While these English words emphasize the passionate, fervent aspect of the original meaning rather than jealousy, you can see how both meanings stem from the idea of intense, burning emotion. The connection between Spanish 'celos' and English 'zeal' becomes clearer when you notice how the Greek 'z' sound often corresponds to 'c' in Spanish words of Greek origin.