It's explained on this slashdot posting:
http://slashdot.org/comments.pl?sid=75144&cid=6727721

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Chewbacca Defense (Score:5, Informative) by Anonymous Coward on Monday August 18, @07:22PM (#6727721)

South Park creators Trey Parker and Matt Stone are huge Star Wars fans. There have been several Chewbacca references on the show.

        In the "Chef Aid" episode, Chef is accused of trying to steal the song "Stinky Britches," which he really wrote many years ago. The record company takes Chef to court, and they hire Johnny Cochran to prosecute Chef. The whole town is wondering if he will use his famous "Chewbacca Defense," which he used during the O.J. Simpson trial. Here's a transcript:

Ladies and gentlemen of the supposed jury, I have one final thing I want you to consider: (pulling down a diagram of Chewie) this is Chewbacca. Chewbacca is a Wookiee from the planet Kashyyyk, but Chewbacca lives on the planet Endor. Now, think about that. That does not make sense! (jury looks shocked)

Why would a Wookiee -- an eight foot tall Wookiee -- want to live on Endor with a bunch of two foot tall Ewoks? That does not make sense!

But more importantly, you have to ask yourself: what does that have to do with this case? (calmly) Nothing. Ladies and gentlemen, it has nothing to do with this case! It does not make sense!

Look at me, I'm a lawyer defending a major record company, and I'm talkin' about Chewbacca. Does that make sense? Ladies and gentlemen, I am not making any sense. None of this makes sense.

And so you have to remember, when you're in that jury room deliberating and conjugating the Emancipation Proclamation... does it make sense? No! Ladies and gentlemen of this supposed jury, it does not make sense.

If Chewbacca lives on Endor, you must acquit! The defense rests.

Later in that same episode, Cochran has a change of heart and defends Chef when Chef sues the record company. Again, he uses the Chewbacca Defense, although with some minor changes:

Ladies and gentlemen of this supposed jury, you must now decided whether to reverse the decision for my client Chef. I know he seems guilty, but ladies and gentlemen... (pulling down a diagram of Chewbacca) This is Chewbacca. Now think about that for one moment -- that does not make sense. Why am I talking about Chewbacca when a man's life is on the line? Why? I'll tell you why: I don't know.

It does not make sense. If Chewbacca does not make sense, you must acquit!

(pulling a monkey out of his pocket) Here, look at the monkey. Look at the silly monkey! (one of the juror's heads explodes)

Eventually, Chef wins the case and all is well.