Green’s Dictionary of Slang

knackers! excl.

a general excl. of derision or dismissal.

[UK]J. Curtis There Ain’t No Justice 142: ‘Lay off of that. You’re spieling knackers.’ ‘Knackers to you and all.’.
[UK]P. Larkin letter 20 Dec. in Thwaite Sel. Letters (1992) 112: Yours with best wishes for the ‘festive season’ and heartiest ‘good luck’ for a prosperous new year knackers Philip (Drawing of hand giving two-finger sign).
[UK]C. Wood ‘John Thomas’ in Cockade (1965) Act I: Knackers.