Green’s Dictionary of Slang

knickers! excl.

also knickers to you!
[? euph. for knackers n. or use of knickers as a juv. ‘obscenity’]

a general excl. meaning rubbish!, you must be joking! etc; general negation of the preceding speaker’s opinion, demand etc.

[UK]P. Theroux Family Arsenal 117: ’Knickers!’ she cried, bringing out a tray of burned scones.
[UK]J. McClure Spike Island (1981) 429: [She would] bend over with her bottom towards the parade, and lift her skirt to show her big green knickers. You know, ‘Knickers to you lot!’.