Mouthbreathing is the act of inhaling through one's mouth rather than through the nasal passages. It is alleged by some looks theorists, as well as professional orthodontists, that mouthbreathing as a child or adolescent can lead to poor jawline development, crooked teeth, and various other facial issues.
Causes of mouthbreathing
People typically develop a mouthbreathing habit when they have chronic nasal congestion, which can be due to several causes such as allergies or a significantly deviated nasal septum.
As a slur
The words "mouthbreathing" or "mouthbreather" are also used as insults along the same lines as "moron" or "retard", which is pretty ableist considering that many mouthbreathe only because they have medical concerns that impede nosebreathing.