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 05, 2006
Those annoying fucking pop ups that go behind your window all the time
Pop unders sure do suck balls
by One Eyed Milkman May 01, 2004