Green’s Dictionary of Slang

(Aus.) stupid, foolish.

1938		H. Drake-Brockman Men Without Wives Ii i: Mrs Abbott and her sonky ideas!
1945		Baker Aus. Lang. 130: Drongo and sonky, mean silly or foolish.
1958		F.J. Hardy Four-Legged Lottery 93: ‘That sonky thing,’ she replied with all the scornful pride of a beautiful young woman. ‘When I’m courted it will be by a real man.’.