Slim Mints are a confectionery product sold by girl scouts.
First shown in the episode Papa Don't Leech, when Lisa handed it (along with other cokies) to mayor Quimby. She expected him to pay for that, but he was never going to. So she has to take it away from him by force.
Second appearance happened in Homerland. When Homer is interrupted by Lisa in his plot to sabotage the Springfield Nuclear Power Plant. Lisa says that "girl scout cookies get you anyway" after he asked how he got it.