Cats don’t wear pajamas unless their fruitcake owners make them.
The expression "It was the cat's pajamas!" was in vogue, I believe, in the early 1900s, meant to mean the very best of something. Kind of like another expression of the same era meaning the same thing, "the lobster's dress shirt." Driving that Maserati was the lobster's dress shirt!
Of course, lobsters don't wear dress shirts unless ... well, you think of something.