Green’s Dictionary of Slang

knobhead n.

[knob n. (1d) + -head sfx (1)]

(orig. US) a stupid person.

[US]Irwin Amer. Tramp and Und. Sl. 118: Knob Head.- [...] a stupid individual.
[US]Ragen & Finston World’s Toughest Prison 806: knob head – A stupid individual.
[UK]K. Sampson Powder 251: Fuckinell! Knobhead’s coming over!
[UK]N. Griffiths Stump 12: — Knob’ed. Get the fuckin bus, lad.
[UK]ShortList (London) 22 May 7: To be honest, we thought he must be a bit of a kn*b-head.