Meaning

noun a member of an ancient Jewish sect in Judea in the first century who fought to the death against the Romans and who killed or persecuted Jews who collaborated with the Romans

noun a fervent and even militant proponent of something

originInitially only found as Middle English zelote, an epithet of Simon the Zealot, acquiring its current senses in the 16th and 17th centuries. The Middle English derives from Latin zēlōtēs, from Ancient Greek ζηλωτής (zēlōtḗs, “emulator, zealous admirer, follower”), from ζηλόω (zēlóō, “to emulate, to be jealous”), from ζῆλος (zêlos, “zeal, jealousy”).

Etymology from Wiktionary, CC BY-SA 3.0