The characters literally combine to mean “stuffy-stuffy-not-happy.” This paints a powerful picture of someone whose emotions are so suppressed or heavy that they cannot experience joy.
闷闷不乐 (mènmèn bùlè) reflects a cultural nuance in expressing negative emotions. In many East Asian cultures, there is a greater emphasis on emotional restraint compared to the more overt emotional expression often valued in the West. This idiom perfectly captures a state of sadness that is felt internally and observed by others, rather than loudly proclaimed. It's a quiet, personal gloominess. To compare, an American might say, “I'm feeling really down,” or “I'm in a funk.” These phrases describe a similar mood. However, 闷闷不乐 is unique because of the character 闷 (mèn). It carries a physical sensation of being “stifled” or “weighed down,” as if one's spirit is trapped. It's not just an absence of happiness; it's the oppressive presence of low spirits that keeps joy out. It's a condition often met with gentle inquiry (“What's wrong?”) rather than a demand for an explanation.
This idiom is very common in everyday life and is understood by everyone.