1. Someone who is really dumb
2. A person who doesn't/can't breathe through their nose

The word Eleven in Stranger Things uses to call someone a Bullshitter.

The actual meaning of Mouthbreather is someone who has a deviated septum and therefore has trouble breathing through the nose. But people misunderstand the reason that they breathe through their mouths and misintepret it as an insult and don't realize that it is not their fault.

Golden State Warriors' Draymond Green

A person who will wear their face mask over their mouth, but not their nose.

Someone who breathed through their mouth instead of nose when they were younger, cussing their bones to mutate and making heir jaw and face look stupid and unattractive.