Open Mouth Breather - An individual that breathes through their mouth when not exerted. The individual appears to be, and ususally is, perpetually confused, not very bright, an idiot.
You see someone from Wisconsin driving along. As you pass them you see their profile and their mouth is wide open, an Open Mouth Breather. Although they are looking straight ahead, they don't see traffic stopping and slam their brakes on at the last moment. This is an example of an OMB (Open Mouth Breather)
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