Complete dumbass that finds it difficult to even breathe.

Someone who's been in too many fistfights and thus has had the ability to breathe through the nose beaten out of him. The usual implication is that the ability to use the brain has also been beaten out of him.

A Conservative.

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

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 mechanical engineer who is much inferior to their counterpart the electrical engineer. Often found staring at a machine with mouth agape breathing through their mouth in a stupid manner and blank expression on their face wondering why the machine will not work. Luckily an electrical engineer is never far away to save the day for the mouth breather.

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.