What does "the cat's pajamas" mean — and why is it funny?

informal, playful

Meaning

Something or someone wonderful, stylish, or first-rate.

Where it comes from

A piece of 1920s American slang, from a craze for nonsense phrases pairing animals with unlikely possessions.

Why it is funny

The humor is the image of a cat owning, and wearing, pajamas. It is glamorous, absurd and completely pointless — exactly the playful, jazz-age silliness the phrase was built for.

Used in a sentence

"She got a new convertible and thinks she's the cat's pajamas."