I have bad sinuses. I have allergies. I frequently have to breathe through my nose and mouth at the same time. And yes, sometimes I breathe through my mouth.
Is there some reason why people who have trouble breathing through their noses are derided as lesser than those who can breathe through their noses?
So, fuck ya’ll.
Jerks.
Careful, don’t keep your head held high in indignation too long while you’re walking…You might fall off the side of the Earth. 
When people have bad breath and breath through their mouths, it just makes it that much worse.
Here, have a mint. No, I insist. Chew the fucking mint or stop breathing near me.
Yes. It’s because they look stupid with their mouths hanging open.
Guilty!
Sorry dude. Seriously. Have you tried Mucinex?
The term came into use because many mentally retarded people habitually have their mouths hanging open (often due to the structure of their jaws/tongue), often with the tongue protruding.
No one notices if someone normal-looking has to breathe through their mouth. I’m sure you don’t go around slack-jawed.
When I hear the term “mouth breather”, I picture a neanderthal-ish character standing there with an open mouth, breathing noisily while staring emptily at whatever it is that means nothing to them. I think pop culture and movies have propagated this image.
Don’t get your panties in a bunch. Just because there’s not an absolute correlation doesn’t mean one doesn’t exist. Besides which, it’s generally clear when people are breathing through their mouth due to nose issues versus the slack-jawed look of idiots.
The issue of slack-jawed mouth breathing is an interesting one, though, since it really does seem to correlate strongly with idiots. Makes me wonder what the mechanism is.
I find that it works even on a transient scale amongst non-idiots… I notice sometimes if I’m just waking up, and my brain is still half asleep and I’m not operating at full brain power, I’m far more likely to have my mouth open.
Some folks’lll never eat a skunk
But then again, some folks’ll
Like Cletus, the slack-jawed yokel
How about we replace it with “open-mouth food-chewers?”
Eh, I also have permenant allergies and have to breathe through my mouth a lot.
The expression has never bothered me.
I’d be up for replacing it with "Lip-movers" to designate how dense guys read books.
I mostly notice the open mouth breathing in correlation with advanced age (70 plus) and a lack of teeth. I’m not sure if there is a physiological reason for that, but I assume that there is.
I am less willing to assume that about the tendency to tell endless stories of the past, out of the blue, to those who just happen to be in the same place at the same time which also crops up in those interactions.
I am desperate for scholarly research into why so many of those stories happen in locker rooms while the teller is naked, was naked when I came in, and remains naked as I finally extract myself from the “conversation” and get on with my day.
St. Augustine, a serious brainiac in his time, had trouble with that one (and was very impressed that St. Ambrose could read in complete silence and without mouthing the words to himself) 
This reminds me of that time I was in Portugal, and I walked by a man sitting by the side of the street. What was strange about this (strange being how we described these things in those days) was that he had a really strange looking right hand, all lumpy and deformed, which may have been the style at the time. I meant to enquire as to this but was distracted by the sight of Johnny Knoxville walking past me, heading northbound towards the city centre, which is what we called the middle of the city in those days.
That’s a fun factoid. I was suddenly and (quite undeservedly) proud of myself for reading quietly, and I had a fleeting image having passed the fifth year test at Hogwarts of having mastered quiet spells.
My question is always:
“How the fuck can you remember all of the pointless details and boring minutia of that inane event from half a century ago, and not remember the fact that you already fucking told me that awful story - several times.”
That was fucking interesting. Could you work “strange” in there one more time?
This reminds me of that time I was in Portugal, and I walked by a strange man sitting by the side of the street. What was strange about this (strange being how we described these things in those days) was that he had a really strange looking right hand, all lumpy and deformed, which may have been the style at the time. I meant to enquire as to this but was distracted by the sight of Johnny Knoxville walking past me, heading northbound towards the city centre, which is what we called the middle of the city in those days.
Agatha Christie has lots of young female characters–often ones who get killed off–who are supposed to be not very bright and are described as “the adenoidal type”.