In the Netflix series, Stranger Things, there’s a scene where a couple of kids are whispering to each other and giggling as they point to another kid and say, “Look, he’s a mouth breather.” 

What they are insinuating is that the other kid – the mouth breather – is stupid, dumb – a knuckle dragger.  This is also how Hollywood always portrays dumb characters, with their jaws slack and mouths hanging open. And there may be some truth to it.