"The Dunning–Kruger effect is a cognitive bias in which unskilled people make poor decisions and reach erroneous conclusions, but their incompetence denies them the metacognitive ability to recognize their mistakes. The unskilled therefore suffer from illusory superiority, rating their ability as above average, much higher than it actually is, while the highly skilled underrate their own abilities, suffering from illusory inferiority.
"Actual competence may weaken self-confidence, as competent individuals may falsely assume that others have an equivalent understanding. As Kruger and Dunning conclude, "the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others". The effect is about paradoxical defects in cognitive ability, both in oneself and as one compares oneself to others."
I'm just learning about this, but it's something I've observed in people for many years. I think my observations started with this guy I knew online who thought he was a great hacker, but he was only a mirc kiddie scripter, and at that time I observed that he had some mental block concerning how tiny his knowledge of such things as hacking really was. And so he was overly-confident of his "l33t" hacking abilities.
Over the years I've observed this same phenomenon in people online and irl. For an irl example, I can NOT convince an INTJ co-worker that he doesn't know enough about some subject that is not in his field of specialty no matter how hard I try. And in the long run, he will simply say he's right because of his social standing. This is called Argument from Authority, but pointing out informal logical fallacies also has no effect since social standing is superior to all reasoning and logic.
Now at least I can give this phenomenon a name. But it would be incorrect to say that such people are 'unskilled,' their confidence is simply greater than their knowledge of a subject.