The simple meaning of when you defy someone in a dream would be to rebel, refuse to submit.
In a dream a reverse dynamic can mean that the individual is being faced with an obstacle and attempts to overcome it. For example, I refuse because I am faced with an obstacle outside of my control. This is also an act of fighting back and standing up for oneself which is making it incumbent on them to confront the system that their beliefs oppose.
Either it is an act of defiance or a person, in the dream you need to beware and understand what are you not accepting.