1. To become filled with mist. The pasture often misted up as the cool night gave way to the warmth of the morning.
2. To become covered in or obscured by condensation. The windows misted up in the early hours of the cold morning. My glasses misted up as I came in out of the cold air.
3. To cause something to become covered in condensation. In this usage, a noun or pronoun can be used between "mist" and "up." The warm air of the shop misted my glasses up as I stepped in from the cold. My breath misted up the windows of the car as I sat waiting in the parking lot for Lisa.
4. Of eyes, to fill with tears. It broke my heart to see my mother's eyes mist up as I said goodbye.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.