Title of the 17th episode of the 2nd season of the animated television comedy South Park, the underpants gnomes were a secret organization of gnomes who repeatedly stole underpants belonging to a beleagured child named Tweak. A clever jibe at failed dot-coms, the gnomes had a plan for world domination that had three steps, the first of which was to steal underpants and collect them in their underground lair. Unfortunately, they had no idea what the second step in their plan should be. However, they were positive (even irrationally exuberant) that they would achieve world domination (or big profits) in the third step of their master plan.
The rest of the episode was a spoof of the Starbucks/Walmart destroying local business meme.