The easiest way to define "narcissistic injury" is to say it is any attack or wound to the narcissist's ego. That still encompasses a lot of ground, as narcissists are consumed by their own egos.
Narcissism is a psychological condition which was codified by Sigmund Freud and named after the Greek God Narcissus, the man who found it impossible to love anyone else but fell in love with his own image in the water instead. Obviously this condition has to do with an excessive ego, but it is more than that. Some people have more pride than they should or are completely self-serving or greedy. A narcissist takes those things to another level, and everything they do is centered on meeting their own needs to the exclusion of everyone else in their lives.
A narcissistic injury is anything that seems in the least demeaning to the narcissist--and he spends a great deal of his time and energy looking for those things. One narcissist describes it this way:
I am constantly on the lookout for slights. I perceive every disagreement as criticism and every critical remark as complete and humiliating rejection--nothing short of a threat. Gradually, my mind turns into a chaotic battlefield of paranoia.
These injuries can be caused by the loss--or the threat of loss--of anything that feeds his narcissism. They can also be caused by the ineffectiveness of the narcissist's strategies to maintain control or some unexpected situation in which they are forced to face their own flaws.
So, what happens when a narcissistic injury occurs. A narcissist admits the following:
I react defensively. I become conspicuously indignant, aggressive, and cold. I detach emotionally for fear of yet another (narcissistic) injury. I devalue the person who made the disparaging remark, the critical comment, the unflattering observation, the innocuous joke at my expense.
These injuries can drive a narcissist to self-abuse with either drugs or alcohol; however, he is most likely to retaliate with anger and derision toward the perceived perpetrator or really whoever is still around him. He is also likely to seek the vital attention he so craves from other sources.
In short, if a narcissist is injured (which is probably just his perception, remember) he is likely to do two things: lash out at the "offender" and do something to recover the attention he thinks he has lost.