To me, there are a couple of different definitions you could use. One would be more a lack of common sense you often see in stuff like funny videos. Another would be incapable of learning anything at all, regardless of what it is. That you can learn so much, even about only one particular subject or another, doesn't say anything about how smart you are, but only speaks for what you might be interested in. Nobody can ever know everything because nobody would ever have the patience to.
_________________
About suffering they were never wrong,
The Old Masters: how well they understood
Its human position; how it takes place
While someone else is eating or opening a window or
just walking dully along...