Green’s Dictionary of Slang

cat’s pyjamas n.

also antelope’s tonsils, cat’s cuffs, ...lingerie, ...mac, ...nightgown, ...pajamas, ...vest, caterpillar’s camisole, ...kimono, ...spats, clam’s garters, kipper’s knickers, kitten’s vest, lion’s bathrobe, sandfly’s garters
[all constr. with the; coined, like many other similar terms, by the US sportwriter T.A. ‘Tad’ Dorgan (1877–1929)]
[1920s+] (orig. US)

1. anything exceptional, superlative.

2. used sarcastically, a joke, a ‘laugh’.

3. a superior person, or someone who poses as such.