Originally Posted by bluejack
(PS. "mouth breather" is also colloquial for "a stupid person; a moron, dolt, imbecile.")
I think that negative connotation just applies to people who do it when NOT exerting themselves. It's basically simian behavior which is where I'm sure the term originated.
When I'm riding at the brink of my anaerobic threshold, I'm going to be doing some mouth breathing. That's just the way it is.