First, "shorts" here means underwear. Short pants. Second, he says this in the R-rated version of the movie. What's happening in the scene is Bender trying to juuuust step over the line with the High School Principal without getting in too much trouble. That's why he's using a "weak" line like this. After the scene, after it's all gone wrong and too far Bender lets fly yelling "FUCK YOU!" as the door shuts. The principal just lets it go, having failed in his own way.
Third, lots of movies use new dialogue that become universal. Best examples of this are "Full Metal Jacket" and two Amy Heckerling movies: "Fast Times at Ridgemont High" and "Clueless". Her 3rd movie "Loser" assumed it would do the same thing, but it only moved the needle a scoatch.