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1. To become covered in a layer of fog. My glasses fogged up as soon as I stepped out of the ice rink into the August humidity.
2. To cause something to become covered in a layer of fog. In this usage, a noun or pronoun can be used between "fog" and "up." I breathed on the mirror to fog it up.
3. To become emotional and nearly cry over something. Yes, I fogged up during the graduation ceremony—"Pomp and Circumstance" gets me every time.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.