It is hard for me to admit, but I have lived to be 45 years old without properly defining the word "stupid." Recently, I read The Basic Laws of Human Stupidity, and it opened my eyes. It gives precise, almost mathematical definitions of what stupidity and intelligence mean.

An intelligent person does things from which they gain while, at the same time, creating gains for others.
A stupid person does things that create losses for others without gaining anything themselves (and often creating losses for themselves, too).
Simple!

It is a very short book, almost an essay, so I really recommend that you read it yourself because it is very thought-provoking. For example, if sigma is constant in any social group, what does that mean for a group of one? Most importantly, it answers the question: "What do I do in order to be intelligent?" It gives you the tools, and the permission, to be intelligent. And if that resonates with you, I also recommend this book. It explains the best way to behave intelligently. If you realize that the same things have different values for different people, it is much easier to create gains for others and for yourself (i.e., to behave in an intelligent way).