Green’s Dictionary of Slang

knocker n.3

also knocka
[? knucklehead n.]

(US black) a fool.

[US]Boston Globe Sun. Mag. 21 Dec. 7–8: There is [...] the ‘knocker,’ the ‘fruit,’ who is in the same class with the ‘nutfarm.’.
[US]C. Major Juba to Jive.