Stands for "No Joke Zone". Thought up by the author of "Odd Girl Out"
Use it to stay clear of bullies, be sure to use the NJZ term whenever you feel violated. You can talk to your friends about what it means so you can take eachother seriously and only use it when it's a No Joke Zone.
"I hope you get ran over by a car."
"Dude, that was an NJZ"