mouth breather

noun2

1. •
   A person who routinely inhales and exhales through the mouth, instead of through the nose.

   • Respirations cannot be recorded by this method, if the subject is a mouth breather (as no air flows by the thermistor in the nose). — 1967
2. •
   (slang, offensive) A person who is boorish, stupid, or otherwise unattractive.

   • 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. — 2004