I must begin by saying that Chewbacca did nothing wrong.
The term Chewbacca defense is, as you'll be able to tell from the spelling, an American one. It originates in a 1998 episode of the cartoon series South Park, where the defense attorney deliberately tries to confuse the jury by focusing his argument on an irrelevant circumstance. In this case, it's why Chewbacca would want to live on Endor with the much smaller Ewoks. The defense attorney says that this does not make sense, and for this reason the jury must acquit the defendant.
What's Chewbacca got to do with it? Not a lot.
This episode of South Park was satirising the trial of OJ Simpson. A glove had been found at the scene of the murder which may have been dropped by the murderer. The glove didn't fit OJ Simpson, and therefore, the defense attorney argued, OJ Simpson couldn't be the murderer. The punch-line was 'If it doesn't fit, you must acquit'.
Most matters of logical argument were dissected several centuries or millennia ago and tend to have names in learned tongues, so it's rather sweet to have this one named by a cartoon show after a character in a Sci-Fi film series.
Well, it's either sweet or completely appalling.
And what happened in the trial of OJ Simpson?
Well, no jury could possibly fall for such a trick.
Word To Use Today: Chewbacca. Although Chewbacca sounds like chew tobacco, and even though in French the character was originally named Chiktaba because it sounded more like the French tabac à mâcher, or chewing tobacco, in fact the name derives from sobaka, which is the Russian word for dog. (George Lucas had a very hairy upright dog who inspired the character of Chewbacca.)