Lisa D writes in and asks,
It's a silly question, but one we have to admit that we didn't have an immediate answer for. After checking around, we learned that the phrase originated by Tad Dorgan in the 1920s. During that time it was considered hip and cool to use jazzy phrases. Dorgan created a number of original phrases and used them in the comic strips he created for publications like the New York Journal.
If something was declared to be "the cat's pajamas", it was being declared to be the best of the best. It's unclear whether the cat's pajamas are better or worse than the bee's knees, but we do know that no cat wants or needs to wear pajamas.