1. literally, someone who lacks enough intelligence that they never learned to breathe through their nose.

2. a really dumb person.

someone who is ignorant to the social courtesies of public movement. consequently, they stifle the mobility of thinking humans by becoming an obstacle which must be negotiated. they can be identified by their despondent facial expressions and wide-open mouths .

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.

anyone who you see with their mouth wide open, like a booger has clogged their nasal orifice. They just look like they're dumb, doing everything with their mouth open, with no care in the world.

Derogatory. Special species of fanboy dork who breathes through his mouth. When not collecting funny books, watching cartoons, playing dungeons and dragons, or video games, he is stalking goth girls or getting beat up.

A person who breathes loudly through their mouth with a nasally voice. Generally seen playing Dungeons and Dragons in the school library and looking at pictures of naked anime women in the computer labs.
Studies indicate that mouth breathers also have loud obnoxious laughter.

a fat person who cant breath through their nose because walking is too much excercise for them