adjective · low-pitched, dull, or stifled (of sound)

远处传来了沉闷的雷声。

adjective · feeling low, unhappy, or lacking in spirit

他这两天心情沉闷，一句话也不想说。

adjective · oppressive or heavy (of weather, environment, or atmosphere)

窗外天空阴暗，天气沉闷得让人感到有些喘不过气。