Internet popup ads that appear and automatically minimize. Even more annoying than popups, because you have to open the image and then close it.
I'd been browsing sites for several minutes and hadn't looked at my system tray. When I did, I noticed I had 42 popunders.
by Ben Frey April 5, 2006
Get the popundermug. by Craigslist Hero July 27, 2011
Get the Popundingmug.