You mainly say this when someone wants you to do something, but you refuse to do it. For example: Friend: Hey I need to borrow $500 to ___(do something stupid)___. You (laughing at him): Yeah...um... that's not happening. ---------------------- You can also use it when someone had been told something would happen but now it's not. Friend: John said he was going to take care of everything. You: Well, it's not happening.

Two meanings, one of which is the meaning that you give it: If A has already happened, but nobody expected it or it is considered to be something awful, people would express disbelief: "The opening act just phoned. Their flight got cancelled." "This is NOT happening!" If A has not happened and I wish to say that it will not be allowed to happen; "Our daughter wants to stay at an overnight party with boys." "Well, it's not happening. Tell her no."