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.