Green’s Dictionary of Slang

1. (Aus.) an Asian.

1973

Tharunka (Sydney) 26 Oct. 11/1: In 1856 gold was discovered, the word ‘slaphead’ was invented (people took racism a lot more seriously in those days and slaughtered non-Anglos en masse, now it’s just institutional).

2. a bald person.

1989		A. Ahlberg Heard it in the Playground (1991) 100: Jug ears! / Slap head! / Four eyes!
1999		Guardian 14 May 30: The Aston Villa midfield, or as they are better known in football circles, the Villa Park slapheads.
2002		G. Evans Dancing Shoes is Dead 277: A large, avuncular, jug-eared slaphead.