Wiktionary
mouth breather
alt. 1 A person who routinely inhales and exhales through the mouth, instead of through the nose. 2 (context idiomatic slang derogatory English) A person who is boorish, stupid, or otherwise unattractive. n. 1 A person who routinely inhales and exhales through the mouth, instead of through the nose. 2 (context idiomatic slang derogatory English) A person who is boorish, stupid, or otherwise unattractive.
Usage examples of "mouth breather".
He missed his first chance at space-suit drill because a battered nose had turned him into a mouth breather-the respirator for a type-M suit calls for inhaling through the nose and exhaling through the mouth.