Green’s Dictionary of Slang

(Aus.) to spit.

1965		W. Dick Bunch of Ratbags 238: He cleared his throat and spat on the car grille. ‘Hell,’ muttered Richie, ‘he’s slaggin’ on me car!’.
1985		Bulletin (Sydney) 8 Oct. 148/4: ‘Where do I stick it in?’ asked the former minister for the Yartz, wielding the knife. ‘Jesus,’ he laughed, ‘I’ve slagged all over it!’.