Green’s Dictionary of Slang

fair dinkum adj.

(Aus.) completely honest.

1880		Maitland Mercury 15 Jan. 4/6: True Blue gives promise of doing something this meeting, as he put Lowlander through on Saturday morning over a mile, doing the fair ‘dinkum’ with whip and spur.
1891		West Australian 6 Oct. 2/2: Shaw said to complainant, ‘Go on, you are a fool; can’t you stand a joke?’ Complainant said, ‘Fair dinkum?’ Shaw said, ‘Yes.’ .
1894		Star (Canterbury, N.Z.) 12 Dec. 3/2: Australian slang is the weirdest in the world. ‘Fair dinkum’ in that lingo means a real Australian.
1906		Sun. Times (Perth) 23 Sept. 4s/4: Wot I got ’ere is fair dinkum stuff, about the Fremantle Prison.
1907		N.Z. Truth 30 Nov. 2/3: He could see I was fair dinkum.
1910		Register (Adelaide) 4 Jan. 10/3: You had me ‘square dinkum’ at the Morphetville races, but you couldn’t find the square dinkum bloke.
1912		Sun. Times (Perth, Aus.) 25 Feb. 14/1: ‘Aw, you’re stuffing us,’ declared Douglas. ‘No, that’s fair dinkum,’ assured Mr Bateman.
1914		E. Dyson Spats’ Fact’ry (1922) 65: Well, I’m blue-blinded [...] what price fair dinkum ’n’ square dealin’ in woman after that? [Ibid.] 89: Goldmighty! [...] whatsa matter with the offer? It’s fair dink, I tell yeh.
1919		Cairns Post (Qld) 23 Dec. 4/6: Almost too good to be true — but it’s a square dinkum fact!
1922		T.E. Spencer ‘Liza’ in Budgeree Ballads 83: It’s fair dinkum what I’m tellin’ yer.
1930		L. Lower Here’s Luck 78: ‘All this chop and tomato stuff - is it fair dinkum?’ ‘It is,’ he replied.
1944		L. Glassop We Were the Rats xi: I couldn’t believe it was fair dinkum. I thought one of the boys was having a joke.
1947		Sydney Morn. Herald 19 Feb. 11/2: ‘No kidstakes?’ ‘Square dinkum I am. You’re a cobber of mine so it’s square dinkum when I tell you I’m saving up for a bike’.
1956		T. Sutherland Green Kiwi 126: No, that’s fair dinkum – without a word of a lie.
1959		Record Hackensack (NJ) 8 Sept. 45/2: [In] a special section of Australian slang I learned [...] that instead of being a ‘dabster’ (expert) I am only a ‘dag’ (humorous fellow) who is ‘giving a fly’ (making an attempt) at talking ‘fodder’ (nonsense) [...] I assure that this is all ‘square dinkum’.
1965		F.J. Hardy Yarns of Billy Borker 13: Did I ever tell you about the only fair dinkum raffle ever run in Australia?
1971		B. Crump ‘Bastards I Have Met’ in Best of Barry Crump (1974) 272: A fair-dinkum crackerjack possum-dog, he reckoned.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 41: I’m fair dinkum [...] a small businessman trying to earn a quid.
1988		McGill Dict. of Kiwi Sl. 36/2: dinkum genuine or fair, often expressed as fair dinkum, dinky-di and dinkum oil (the truth).
1993		Sydney Morn. Herald 25 May 24/4: There’s ‘fair dinkum,’ ‘square dinkum,’ [and] ‘dinkum oil’ .
1999		Guardian 28 Oct. 20: Australians like it fair dinkum [honest].
2005		M.B. ‘Chopper’ Read Chopper 4 171: I mean, these are fair dinkum, ‘She’ll be sweet, mate’ bloody Aussies.
2016		N. Cummins Adventures of the Honey Badger [ebook] I didn’t know if this [i.e. a tour company] was a Mickey Mouse outfit or if it was ridgy didge. Fortunately it was fair dinkum and they were on the case.

In derivatives

fair-dinkumness (n.)

honesty, fair play.

1905

Sun. Times (Perth) 23 July 4/7: And we got to think fair-dinkumness was branded in at birth, / But I’m damned if I’ve discovered much of it about in Perth.