Fatuousness is the quality of being fatuous: empty-headed silliness that looks a bit pleased with itself. It names the thing, not the person. Writers use it when they want to point at a style of thinking — a comment, a speech, a public smile — and say the foolishness is not just a slip, but a kind of hollowness. It is a formal, rather heavy noun; in everyday talk people are more likely to say stupidity or silliness. Though actually, those everyday words miss a shade: fatuousness is foolishness that has no depth, and often no doubt.