Green’s Dictionary of Slang

a general intensifier, added to a n.; thus a grandfather of a...

1885		Bulletin (Sydney) 31 Oct. 22/1: His temper was up, and he had to let out at something. Like the Irishman who, when offended by his wife, fell upon an omadhaun in the kitchen and gave him the ‘grandfather av a batin’,’ Joe, unable to vent his chagrin on any suitable object, took it out on the young ’uns.
1908		Bulletin (Sydney) 15 Oct. 14/2: Treeleaf was willin’, and we went over at the back of the cemetery, and saw Matty get such a gran’father of a hidin’ that he had to be carried home in the chief mourners’ American waggon.