‘Mouth Breather’
Like yeah I don’t get this one. We all breathe through our mouths (and noses), because we have to. Yet I read this sometimes as it is used like an insult. So what are you saying? You’re insulting someone for doing what billions of other human beings have to do naturally?
Calling someone “smooth-brained” always felt backwards.
Like, I understand that brains are supposed to be wrinkly and there’s an actual disorder where the brain doesn’t have those wrinkles, which leads to developmental delays: https://en.wikipedia.org/wiki/Lissencephaly
But it still sounds to me like you’re just calling them “smooth” as in “cool”.