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fog something up
to make something made of glass become covered with a film of water vapor. The moisture fogged the windshield up, and we had to stop to clean it off. The moisture fogged up the glass.
[for something made of glass] to become partially or completely obscured by a film of water vapor. The glass fogged up, and we couldn't see out.
1. To become covered with condensation: The bathroom mirror fogged up after I took a shower.
2. To cause something to become covered with condensation: The steam from the kettle fogged up the kitchen windows. Don't breathe on the lens—you'll fog it up.
3. To become teary-eyed out of a strong sense of emotion: I fogged up when my parents' photo was unveiled.