I guess we all agree that stupid is not just the opposite of intelligent, because we all know some person who is fairly intelligent but also stupid.
So was is the main characteristic of stupid people?
My personal definition is "being unable to think outside the box".
Stupid people are severly blinkered, they are living inside their box (often together with like-minded people!), they know everything about the interior of this box, which is their world, they may be renowned experts about this box-world (yes, there are stupid professors !), but they can't step outside (mentally) to look around and to gather new evidence, maybe even to change their mind.
They may be wonderful persons, funny, nice, empathic, altruistic.... but intellectually their little box is the world, and that's it.
Stupidity: the act of rejecting facts.
Examples include the triumph of ideology over data (sex abstinence as a form of birth control instead of teaching young people about the facts), the misrepresentation one thing as another (claiming that the Bible is the evidence God's existence, not the claim of God's existence), and being terrified of the side effects of Covid-19 vaccines while ignoring the consequences of having Covid-19.
I see stupidity as just one manifestation of cognitive deficiency.
In my opinion, stupidity is making the same type of cognitive errors and decisions repeatedly. Yes, I’ve heard the definition of insanity is doing the same thing over and over again while expecting a different result, but stupidity is a failure to take cognizance of one’s ignorance or shortcomings, ignore new information and thus fail to learn, whether it be from one’s own prior mistakes or the mistakes of others.
Then people discover A Glitch in the Matrix and start “thinking” way too far outside the box with Mandela effects everywhere in sight. So it can go the other way.
But there are post-structuralists such as Baudrillard who inspire us to think outside the box. The Matrix was a bastardization of his ideas.