🔼The name Zealot in the Bible
The name Zealot occurs only twice in the Bible. It's the surname, nickname or epithet of one of the two disciples of Jesus named Simon (the other being Simon Peter). This Simon is called Zealot only twice and only by the Lucan author (Luke 6:15, Acts 1:13). John doesn't mention a Simon other than Simon Peter and Simon Iscariot, and both Matthew and Mark call Simon not the Zealot but the Kanaanite (Matthew 10:4, Mark 3:18; see our article on this epithet Kanaanite, which probably has nothing to do with Canaan).
The Zealots were a group of folks who hated the Romans, and since everybody hated the Romans, it's not always clear where ordinary people stopped and Zealots began. It's also not clear whether the Zealots were actually an organized group, with a structural hierarchy and formally sworn in members and secret handshakes and all that, or whether they were more like modern football hooligans, who wouldn't call themselves such but were dubbed that way by commentators.
Unlike Pharisees, Sadducees and Essenes, the Zealots weren't particularly known for established theologies, but rather for their absurd dedication to wrecking all things Roman (again, probably much like hooligans, with the odd ring leader). There were also militant anti-Roman elements called Sicarii, who tended to stab their victims with a dagger (sicarius; possibly in reference to the assassination of Julius Caesar), but there were obviously also Zealots and surely an untold amount of uncommitted assassins, who too wielded the occasional dagger, so these titles were probably used largely arbitrarily and rather loosely.
The Jewish-Roman historian Josephus (see our article on Dalmanutha) describes the Zealots as a bunch of fiercely cruel and injust morons, who murdered folks of any plumage, including their fellow Jews. During the Great Revolt these Zealots destroyed food supplies and fire wood to force the people of Jerusalem to fight the Romans, and ended up setting fire to the temple and destroying it, while general Titus watched on in horror and dismay.
It's not very likely that Jesus had recruited a person like that for His own inner circle, and it seems that the name Zealot can not solely be applied to one particular modus operandi or even general philosophy. In fact, it seems much more likely that the term Zealot existed besides and long before it was turned into a name and became applied to these senseless guerilla warriors, who in turn most likely evolved during the course of their existence (possibly from the revolt of "founder" Judas of Galilee in 6 AD, see Acts 5:37, to the destruction of the temple in 70 AD).
🔼Etymology of the name Zealot
The name Zealot is the same as the noun ζηλωτης (zelotes), which denotes someone who is filled with or driven by ζηλος (zelos), which literally means hotness and figuratively means fervence or enthusiasm:
The name Zealot literally means Hot Head or One Who's Fired Up and became applied to mean Enthusiastic One and finally Violent Fanatic. By the time of Jesus it possibly had a similar air of pseudo-solemnity as the name Boanerges.