Green’s Dictionary of Slang

(Aus./US) an ungainly, clownish figure.

1904		Leavenworth Times (KS) 14 Dec. 4/3: ‘Sonk’ Gannon [...] fought a former city detective until the latter had worn out a club on his head.
1922		C.J. Dennis in C.J. Dennis Collection 118: Them was the days when we could cuss an’ fight. / Even that young sonk, Romeo, could sweal; / When ’e got set ’e’d fairly raise yer ’air.
1959		D. Niland Gold in the Streets (1966) 142: Silly looking sonk. Head like a melon, big feet, shovel hands. King of the cow-bails, possum eater, the pride of Woop-Woop.
1966		H. Porter Paper Chase 131: Her husband [...] upsets the good-clean-fun pattern of an open-air drinking bout at Eagle Point Park by accusing his wife and a sonk of a bank clerk of unchaste designs on each other.