Definition & Meaning of "mouth breather"in English

What is the origin of the term "mouth breather" and how is it used?

"Mouth breather" is a slang insult that appeared in English in the mid-20th century, originally referring to someone who breathes through their mouth instead of their nose, often implying poor health or laziness. The term evolved metaphorically to describe a person who is stupid, dull, or uncultured, emphasizing perceived mental slowness or ignorance. It is used in informal, often mocking or hostile contexts, such as conversation, media, or online commentary, to express contempt. The term is clearly derogatory and intended to insult; it is never neutral.