What Is Common Sense?

"Sure he's smart, but he's got no COMMON SENSE." Man, I hate that.

What is this "common sense" of which they speak? How do I know for any given sentence X whether X is common sense? Does that mean it is simply widely known? Is it self-evident? Is it a pre-theoretical truth, that is, something whose justification requires only everyday experience and no general theory?

Is common sense unrevisably true? If something is common sense, could it be wrong? If so, what good is it?

Those of you who supposedly have common sense, what is it?