Green’s Dictionary of Slang

cat’s whiskers n.

also bullfrog’s beard, canary’s tusks, cat’s knee-knuckles, clam’s cuticle, crocodile’s adenoids, frog’s eyebrows, lily’s whiskers, oyster’s eye-tooth, pig’s whiskers, snake’s eyebrows
[var. on cat’s pyjamas n.; all constr. with the; later use is mainly Aus.]
[1920s+]

1. anything exceptional, superlative.

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