‘Mouth Breathers’ & Other Putdowns

Dennis: I went over to the Press site this morning to see what the “Mouth Breathers” had to say about this development. The first thing I noticed was the lack of comment on the article. three comments to be precise. What I also noticed was that of the three, 2 were ready to throw Cliff Hayes under the bus. I don’t know about the rest of you, but if this whole recall mess weren’t so sad, it would be hilarious sitcom material.

Question: I know that “mouth breathers” is a put down. But for the life of me I can’t figure out what it means. Anyone? (And/or: What’s your favorite cyber putdown?)