Etymology is the study of the history of words and how their format and meaning evolve over time. According to the Online Etymology Dictionary, the word "zealous" dates back to the 1520's. This is the earliest year there is a written record of the word but it may have been used in conversation years before showing up in a manuscript. Zealous derives from Medieval Latin zelosus. This is Latin written and spoken between the years 700 - 1500 A.D.
It can be compared to Italian, the Romance language spoken in Italy, zeloso: and Spanish, also known as the Castilian, Romance language spoken in Spain and Spanish America, celoso.
Zealous comes from the noun zeal which came from the Latin zelus meaning zeal or emulation. It did the rounds of the Romance languages in, IT zeloso, and SP celoso. It was a Church word and in the Greek it was zelos meaning zeal, ardor, jealousy. In English ‘zeal’ has undergone many additions that alter its usage slightly but the root word is the same. The addition of the suffix ous makes zeal into the adjective zealous which can modify the noun ‘person’ e.g. “A zealous person”. Another usage of the root word is with the suffix ot, making it a personal noun. Simon the Zealot of the Gospels belonged to that party of Jewish Resistors of Roman occupation. Zealous can also take the suffix ness making it again a noun.