Green’s Dictionary of Slang

a general excl. of derision or dismissal.

1937		J. Curtis There Ain’t No Justice 142: ‘Lay off of that. You’re spieling knackers.’ ‘Knackers to you and all.’.
1945		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).
1963		C. Wood ‘John Thomas’ in Cockade (1965) Act I: Knackers.