What is stupidity?

I think stupidity is a word used by arrogant people. One might think a stupid person can be someone that doesn't learn from his/her mistakes, and by mistakes I refer to an action that is detrimental to that person.

But then again, life is a bit pointless, so being selfish and careful is kinda stupid, and that kinda negates the aforementioned definition of stupid. And someone might find comfort in playing it safe, others might not.

The term "stupid" is used to describe someone that doesn't follow your logic. E.g. drinking alcohol and going clubbing might seem stupid to some, while studying math and not living to the fullest is considered stupid by others.

-Living safe VS YOLO.
-Saving money VS money is meant to be used.
-Planning whole life for the retirement VS living out your youth and being in the moment.

I think someone who calls another person stupid is actually showing his/her limited capability or inability to understand difference in people, thus actually pointing out that he/she him-/herself is stupid.

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UPDATE(8.4.2017): something that I just came across.. Paraphrased for your convenience.

"What kind of a scientist doesn't know that molten metal isn't magnetic?"

The purest kind, perhaps? Isn't a part of being a scientist admitting one's own "stupidity"? I mean: you wouldn't be testing stuff if you already knew what's going to happen. I think it's the unknown, the endless possibilities of how things might turn out or what you might learn from them, that drives people into experimenting stuff. It's like the mind of a child: everything fascinating for the very reason of not knowing about them, the more you know the more boring and predictable life gets(I'm not advocating anyone to stop learning, though). 

I've always wondered about people's need for one-upping one another with their own personal knowledge of things to feel somehow better or superior about themselves. I think it's a bit futile: The guy on the video, has been busy learning something else when some of us were learning about magnetism, and it was just now his time to learn this particular piece of knowledge. The guy might know many other things, that most of us don't know yet. Everyone's better at something than someone else, life is like a huge multilayered rock-paper-scissors(-lizard-spock) game. In the end, he's enjoying what he's doing, isn't that all that matters?