Green’s Dictionary of Slang

(Aus.) the honest truth, true facts.

1915		Sydney Morn. Herald 2 Oct. 9/5: ‘Dinkum Oil‘ Trench Newspaper [...] No. V, vol. 1, of ‘Dinkum Oil’ war news, June 23, published in the trenches on the Gallipolli Peninsula by the Ausralians.
1918		Aussie (France) 4 Apr. 3/2: An’ if they wants ta argue much, you send him up ta me. / I know the real square dinkum oil. So there yer are! Compree!
1921		(con. WWI) E. Lynch Somme Mud 323: Snow [...] gives us the dinkum oil about the dinner.
1922		N.Z. Truth 29 Apr. 1/5: He may be putting across the dinkum oil.
1934		T. Wood Cobbers 117: Gold. The Dinkum Oil, as they say here. The Real Mackay.
1944		L. Glassop We Were the Rats 79: I’ve gotta give ya the dinkum oil.
1953		I. Hamilton Till Human Voices Wake Us 50: I got to know the fair dinkum oil about the New Zealand prison service.
1955		N. Pulliam I Travelled a Lonely Land (1957) 232/2: dinkum (dinky, dinkum oil) – true or truth.
1968		S. Gore Holy Smoke 42: Not even the Chaldeans – who was sorta court stargazers, like our Met Office – could give him the dinkum oil about ’em.
1971		B. Crump ‘Bastards I Have Met’ in Best of Barry Crump (1974) 266: It was hard at first to believe it wasn’t some kind of act he was putting on, but no, it was the dinkum oil all right.
1988		McGill Dict. of Kiwi Sl. 36/2: dinkum genuine or fair, often expressed as fair dinkum, dinky-di and dinkum oil (the truth).
1999		G. Seal Lingo 55: The terms dinkum oil or the good oil [...] are said by linguists to be a transfer of the use of oil to ensure the efficient running of a machine to the need to have good information in order to fight effectively.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].