For what it’s worth, I’ve only heard this term in my (American) culture for the past 5 or so years.
My first exposure to the term was via internet memes where “mouth breathing” was conveyed as existing only with those who were mentally impaired. Check out the Urban Dictionary link someone else responded to you with in a different comment thread for more context.
I don’t recall ever hearing anything about “mouth breathing” in my society/culture/subculture before exposure to these memes, so I wonder if it’s a newer thing? If so, silly memes really are influencing our culture!
My first exposure to the term was via internet memes where “mouth breathing” was conveyed as existing only with those who were mentally impaired. Check out the Urban Dictionary link someone else responded to you with in a different comment thread for more context.
I don’t recall ever hearing anything about “mouth breathing” in my society/culture/subculture before exposure to these memes, so I wonder if it’s a newer thing? If so, silly memes really are influencing our culture!