The word "stupid" is defined as: "lacking intelligence or common sense." As a smart mouthed, snot-nosed kid, stupid held the same meaning as a multitude of other insults: dummy, stinky, booger-brain, fart sniffer, etc. Everything and everyone was stupid. I think that this mentality started to take hold around Kindergarten. It was said at least once in every sentence out of my mouth until about 5th grade, when everyone moved on to more mature insults. (If "mature insults" are even a thing.) I believe that "stupid" is such a popular insult for that age group in particular because most parents think of it as a word that isn't "bad." It just isn't nice.
Everyone my age was saying it and no one really thought about what they were really saying, it was just a silly little insult. For a reason that was unclear at the time, my mother absolutely despised being called stupid. I couldn't figure out why it bothered her so much, but now I realize that it was the same as telling her that she was unintelligent and incapable of understanding. Think about it. When someone calls you stupid, they are saying that you are mentally deficient and incompetent. That's offensive.
You can insult my outer appearance, but how dare someone insult my mind? Your looks, taste in music, or the sound of your voice when you speak can all change, but you only get one mind. Yes, your mind can also change. It can grow, if you prefer, but you can never just run down to the supermarket and pick up a new one. I'll be the first to admit that there are many subjects that I don't know anything about, including: punctuation, actinobiology, current video games, or hydroponic farming. However, I am not mindless. When someone says that you are "stupid," they are saying that they do not believe in the capability of your mind. They do not believe in your ability to gain and use new information. I believe that someone's mind is what makes them who they are, not material things like the clothes that they put on their body or how they earn an income. An insult on the mind is a very potent insult and it should not be said lightly. So, please, call me uneducated if you must. At the very least, I can be educated. However, do not ever doubt the power of my mind.