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

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

2. a really dumb person.

1) Person incapable of their own thought, who when faced with a decision, or when asked about personal opinion on something opens up her or his mouth in shock and astonishment that there is such thing as personal opinion. With mouthbreathers it happens constantly due to damage caused by upbringing by helicopter parents, or other emotionally damaging authoritarian or social influences. Can be spotted at every step of social and economic status ladders.

2) A reason why normal people invented law of the land ( equivalent of pinning a note that states "anyone who will touch my coffee cup will get their car keyed" ) and lawyers to herd those idiots away from harming normal families.

3) Equivalent to an alarmist or a zombie - AVOID AT ALL COST - prolonged exposure might lead to compromise of inner emotional barriers and contamination of own soul with emotional AIDS. If you really want to help such person you can only put some objects in their way in hope it will trigger aha moment in them, so they will start slowly doing stuff for themselves - pretty much the same deal as when handling alcoholics

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 .

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.

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.

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.