Down in the mouth 意為「不高興的；悶悶不樂的；心情不好的；神情沮喪的」。這個成語首次出現在 1600 年代中期，指的是人一不高興，眉頭就會皺起來，嘴角向下彎曲。Down in the mouth 本身當形容詞用，通常放在 BE 動詞後面，有時被寫成或說成 down at the mouth。
例句：
- What’s wrong with Sam? He’s so down in the mouth about everything. (山姆怎麼啦？他對每件事都不高興)
- Randy has been down in the mouth ever since Amy broke up with him. (自從艾美跟他分手後，蘭迪一直悶悶不樂)
- Linda’s been moping around all week, and we have no idea why she’s so down in the mouth. (琳達整個禮拜都悶悶不樂，我們都不知道她心情為何這麼不好)
- John looks a bit down in the mouth. (約翰看起來有點神情沮喪)