What, exactly, is "common sense"?

So what is this mysterious “common sense” exactly, that many people toss around, especially the supposed lack of, in more intelligent people?
The catch phrase of “Oh, he/she is really smart but they have no common sense.” I’d assume that if one doesn’t walk around with their head tilted upwards into a rain storm, keeps their fingers out of light sockets, doesn’t play with matches and looks both ways when they cross the street, one could say they have some measure of common sense. What is your definition of “common sense” and why is it that it seems to be the catch all phrase that the less intelligent like to throw around to appease their own deficiencies of intellect?