Mouth Breather Meaning

(idiomatic, slang, derogatory) A person who is boorish, stupid, or otherwise unattractive.

Example: A person who routinely inhales and exhales through the mouth, instead of through the nose.
1967, Nancy J. Barron, "A Ride in the Human Centrifuge," American Journal of Nursing, vol. 67, no. 8, p. 1654,
  Respirations cannot be recorded by this method, if the subject is a mouth breather (as no air flows by the thermistor in the nose).
2004, Tim Goodman, "Trinkets are spirit guides in Fox's brilliant 'Wonderfalls'," San Francisco Chronicle, 12 Mar.,
  She's a philosophy major from Brown, now working retail at Niagara Falls, living in a trailer and working for a "mouth breather" of a boss.