A stupid person doesn’t have much intelligence or imagination, and they go through life making decisions that seem to lack all common sense. If you’ve got a brain but you don’t use it, you might be a bit stupid.
The word comes from a Latin adjective that means “amazed or stunned,” and stupid people are stunned by everything because their minds are numb. You can make a stupid mistake when you’re not thinking, and you can call someone stupid if you’re feeling cruel, like “Hey, Stupid.” Zombies are stupid because their brains are dead. If that makes sense to you, then you’re not stupid, and you’re also not a zombie. Well done!