Someone who keeps their mouth open, often with tongue sticking out; appearing mentally deficient. Implying the person has brain damage or is mentally deficient.
All his friends were idiots and morons; the biggest collection of open-mouth breathers I've ever seen.
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)