Green’s Dictionary of Slang

1. (Aus.) work, esp. hard work, a due share of work.

1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 30: It took us an hour’s hard dinkum to get near the peak.
1941		Baker Popular Dict. Aus. Sl. 23: Dinkum, hard work or honest toil.
1986		G.A. Wilkes Exploring Aus. Eng. 14: Miners in Derbyshire, for example, used dinkum to mean ‘work’, especially hard work [...] From meaning ‘strenuous effort’, dinkum came to mean ‘genuine’ or ‘authentic’, and gained a much wider currency in Australia than it had in the English county from which it came.

2. (also dincum) the truth.

1906		N.Z. Truth 4 Aug. 4/7: Then I told him the straight dincum.
1915		C.E.W. Bean Anzac Book 56/1: I was on the beach one day when a friend met me and asked if I had heard the latest dinkum.
1919		Aussie (France) XII Mar. 1/1: They separated it from the ice, thawed it out, and delightedly absorbed the retrieved liquid joy. That’s dinkum.
1929		Queenslander (Brisbane) 28 Nov. 6/2: ‘A friend? Is that dinkum?’ ‘Certainly’.
1942		Western Mail (Perth) 4 June 2/2: He seems to be a cross between a squarehead and a Jap.
1955		N. Pulliam I Travelled a Lonely Land (1957) 232/2: dinkum (dinky, dinkum oil) – true or truth.
1959		D. Niland Big Smoke 211: Why didn’t I take all your money? [...] Why did I only take sixpence if it wasn’t dinkum?
1977		(con. 1941) R. Beilby Gunner 167: Why would I want to have you on? It’s dinkum, I tell you, the good oil.

3. a loved one, a ‘best girl’ or ‘best boy’.

1912		Sport (Adelaide) 19 Oct. 13/2: They Say [...] That Shan’s dinkum, Mickie, has turned him down.
1913		Sport (Adelaide) 15 Feb. 11/4: They Say [...] That Alick N is not true to his dinkum; he was seen with another tabby.
1914		Sport (Adelaide) 10 July 4/3: Annie C. looks very downhearted since her dinkum went away .

4. an Australian, spec. an Australian soldier in WWI .

1918		Aussie (France) 18 Jan. 3/1: ‘And how often do you get to leave Australia?’ asked the inquisitive old lady. / ‘Once every war,’ replied one of the dinkums; ‘at the end of it.’.
1918		Kia Ora Coo-ee 15 Apr. 2/2: Cooking for a mob of Dinkums is about the most thankless job that I know of, and in my time I have tackled all kinds of employment.
1993		(con. WW1) Sydney Morn. Herald 25 May 24/4: The second shipment of Anzacs were called The Dinkums; the third the Super-Dinkums.

5. something genuine.

1984

P. Corris ‘Stockyards at Jerilderie’ in Heroin Annie [e-book] That made three Castletons, two fakes and a dinkum.