We have some curious phrases surrounding cats.  "Look at what the cat dragged in" "Curiousity killed the cat" and "The cat's out of the bag" for starters, but perhaps the strangest is the phrase "The cat's pajama's."  One doesn't generally expect a cat to wear pajamas, they sleep so much throughout the day that it would be hard to determine when to put them on.  The saying first came out in the 1920s, but no one can pin down a precise moment of origin.  The Oxford English Dictionary credits the origin to the US and defines it as "the acme of excellence" (isn't acme also a great word?  I think of Wile E. Coyote anytime I hear this, but the actual definition is "the highest point or cumination; the point or period at which something is at its best or most highly developed").  Basically if you or something you do is "the cat's pajamas" it's excellent, and is usually used to refer to clothing or the person wearing it.

Curiously (since I am not a cat, I can afford to be curious), no one is quite sure of the phrase's origin, other than "during the 1920s in the US."  There's no specific first use anyone can seem to cite.  Though during the 1920s silky lingerie that flattered the curves became all the rage.  It was also one of the first times in history we became concerned about the attractiveness of undergarments.  They were so immensely popular at the time its not hard to see how it might be incorporated into the developing slang.  As far as the 'cat' half of the phrase, that's a bit more difficult.  It might be easier to parce if the slang 'cool cat' or its kin were around at this time as well, but that's not the case.  What was popular (though it was waning fast) was Felix the Cat.  There was also a comic character named 'Krazy Kat' during the time.  Perhaps they played a roll.  Disappearing Idioms hypothesizes, in addition, that maybe someone just looked at pajamas, looked at a cat, and thought it made an amusing picture.  Not all idioms have to come from concrete places or real things, after all.  Perhaps its even a combination of the two theories, someone saw pajamas and pictured them on Krazy Kat or Felix the Cat.

No matter the origin, it's a fun phrase that's fading fast, and may soon only be heard when your feline is, in fact, wearing pajamas (take pictures).