1. To flash very brightly and suddenly. The lightning had only just flashed up when the crack of the thunder boomed nearby. The car's headlights flashed up, signaling me that a speed trap was nearby.
2. Of a computer notification or message, to appear on the screen very suddenly. By the time the warnings began flashing up on our screens, the reactor had already begun to melt down. A little message flashed up in the bottom corner of my browser telling me that I had won $1 million.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
flash something up (some place)
to shine a light upwards toward something. Flash your light up into the tree. She flashed up her light at the cat in the tree. Gloria flashed the light up.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.