mouth-breather
n.— «Then there are the mouth-breathers. These folks are usually slack-jawed, thick-tongued individuals who congregate around pornographic movie theaters. It is the latter method that is used in cycling.» —by Mike Keefe Ten Speed Commandments Aug. 4, 1987. (source: Double-Tongued Dictionary)
Further reading
Understanding the varieties of conversational styles can mean the difference between feeling you’re understood and being insulted. “High-involvement” speakers interrupt or talk along with someone else to signal their enthusiasm...