Green’s Dictionary of Slang

(orig. US) a stupid person.

1931		Irwin Amer. Tramp and Und. Sl. 118: Knob Head.- [...] a stupid individual.
1962		Ragen & Finston World’s Toughest Prison 806: knob head – A stupid individual.
1999		K. Sampson Powder 251: Fuckinell! Knobhead’s coming over!
2003		N. Griffiths Stump 12: — Knob’ed. Get the fuckin bus, lad.
2008		ShortList (London) 22 May 7: To be honest, we thought he must be a bit of a kn*b-head.