The kipper's knickers is actually one of a set of nonsense catchphrases from 1920's America. It indicates something excellent, admirable or first-rate - NOT a female who's personal hygeine is in question as posted by iamamanufan on this site.
You think you're the kipper's knickers, don't you?
Smoked kippers; Ace Rimmer's breakfast food of choice, apparently, as he mentions them in his slogan every time he leaves in the British TV series Red Dwarf.