If something that you can see through mists over/up, it becomes covered with a thin layer of liquid so that it is more difficult to see through.

Open the window when you have a shower to stop the mirror misting over. 淋浴时打开窗，免得镜子蒙上水汽。

The steam from the kettle misted up her glasses. 水壶里冒出的水蒸气使她的眼镜变得雾气蒙蒙。

If your eyes mist over/up, they fill with tears.