This past Sunday, FOX aired the umpteenth installment of the "The Simpsons: Treehouse of Horror" Halloween special. (It aired Nov. 3, which seems totally appropriate for a Halloween special...) In the last bit of the show, there was a take on the "War of the Worlds" radio broadcast in 1938, complete with a Depression-era Springfield and an Orson Welles sound-alike. The bit was fairly ho-hum, especially as it turned into a non-too-thinly veiled jab at the War in Iraq. And a non-too-funny one, at that.
But there was one, brief, shining moment of hilarity. As the citizens of Springfield riot in a mass hysteria induced by what they think is an alien invasion, Homer is shooting people left and right for making comments that sound too much like what an alien would say. (This is actually a better social satire, especially given the brutal, nasty midterm campaigning happening as this episode aired.) After Homer shoots and kills Lenny, Disco Stu jumps from screen right to screen left, decked out in a white suit and matching hat and swinging a gold pocket watch from its chain. His one line of dialogue was genius: "Big Band Stu says 23 skidoo!" Then Homer shoots him, too.
After writing that line down for posterity, I got to wondering what "23 skidoo" meant. This wasn't the first time I've encountered the phrase; Conan O'Brien uses it from time to time on his show, and it's usually down in this olde tyme way. So I looked it up, and there's some great information about the saying on Wikipedia, which you can read here.
Essentially, it's an old phrase meaning to leave, possibly deriving from the word "skedaddle." That makes sense that a hip crowd in the '20s and '30s would paraphrase such a non-hip word as "skedaddle" into the more swinging "skidoo." (It sounds like a swing dance move.)
The rest of the Wikipedia entry is worth reading, especially because it includes a section of an 1899 Washington Post article in which the author tracks down the meaning behind some new slang term, and also because the entry tracks some recent uses of the term, from John Prine to MF Doom.