‘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?
What source do you have that links it to down syndrome? When people are dumbfounded, they often open their mouth. One of the definitions of “Slack Jawed” is: lacking intelligence or sophistication.
Also, mouth breathing can impact the development of ones face, where you have less muscle tone and a more elongated face. These traits are sometimes associated with someone with less intelligence. Nowhere do I see anything about the term referencing/originating from Down Syndrome.