It means to stop something from happening or to negate something. In online jargon it's when you encounter a ratchet person for example, who’s hoping for a romantic connection, you may think to yourself, it’s time to "cancel." It’s a less abrasive way of saying, no, this isn’t going to happen.

See also : YNGBT  cancelbot  cancelbunny  cancelmoose  CX  
NetLingo Classification: Online Jargon