Green’s Dictionary of Slang

1. to confuse, to bewilder.

1868		Western Times 24 July 2/6: The remarks on the neglect of ‘distinguished foreigners’ who were ‘shandygaffed’ at Shoeburyness [...] were calculated to create erroneous impressions.
1878		York Herald 2 July 3/5: The Middlesboro’ people were not going to be ‘shandy-gaffed’ by such a lot of those.
1890		Windsor and Richmond Gaz. 8 Nov. 4/4: The jumbled mixture [...] plainly shows the writer to be more familiar with shandygaff than English composition [...] the introduction to the unaccustomed [...] completely upset the shandy-gaffed brain.

2. (Aus./N.Z./UK) to mix things up, to combine two things in a confused fashion.

1887		Athletic News (Manchester) 6 Dec. 4/4: It is very difficult to tell where Ashton ends or Stalybridge or Hurst or Dukinfield commences, the four places are so completely ‘shandygaffed’.
1910		Bulletin (Sydney) 20 Oct. 13/2: Anyway, the arguments about wet sheep are generally not whether they are too wet to shear, but whether they are dry enough, and a good many shed bosses have fallen in by ‘shandy-gaffing’ two flocks, or turning the dry sheep in the shed out among the wet ones in the yards, and then putting the lot back in the pens.

3. to adulterate.

1928		Industrial & Mining Standard 289/3: This test will enable to motorist [...] to ascertain whether or not the petrol he is using has been ‘shandygaffed’.
1934		Age (Melbourne) 17 May 10/4: By the time the petrol reached them there was not very much left to shandy-gaff.