The following is from the soon to be deleted page on Wikipedia.
The cat's pajamas is an English slang phrase coined by w:Thomas A. Dorgan. The phrase means "the height of excellence", became popular in the U.S. in the w:1920s,  along with bee's knees, "the cat's whiskers" (possibly from the use of these in radio crystal sets), and similar phrases that didn't endure: "the eel's ankle", "the elephant's instep", "the snake's hip" and "the capybara's spats".
What's the policy on less than genteel language? Another equivalent phrase is the dog's bollocks, which is probably much more widely used and recognised than the cat's meow.
- ^ http://www.randomhouse.com/wotd/index.pperl?date=20010102
- ^ http://www.urbandictionary.com/define.php?term=cats+pajamas
- ^ Mark Israel, 'Phrase Origins: "The bee's knees"' alt.usage.english FAQ file,(line 4407), (29 Sept 1997). http://www.exw6sxq.com/sparky/aue_related/full_faq.html