Green’s Dictionary of Slang

1. Australia; supposedly sited ‘underneath’ the UK on the globe; also used adverbially.

1899		Shields Dly Gaz. 26 Aug. 3/7: [headline] Bowls ‘Down Under.’ Australian Team Coming.
1900		Bulletin (Sydney) 10 Feb. 24/3: He tried South Africa without success, and since arriving ‘down under’ has had several battles, but won none – albeit he fought a great fight each time.
1915		Honk! 29 Aug. 5: NEWS FROM ‘DOWN UNDER’.
1918		R.H. Knyvett ‘Over There’ with the Australians 83: There were beauties there from many nations [...] who were tickled to death to be escorted by the bronzed giants from ‘down-under’.
1926		J. Doone Timely Tips For New Australians 7: The new arrival in Australia be told in straight forward fashion that he will be credited with knowing little or nothing ‘down under’.
1941		N.Y. Herald Trib. 29 June 9/2: The Australian doesn’t mince words, and his characteristic expressions often reveal [...] the colorful vigor of life on the continent ‘down under’.
1943		Army News (Darwin, NT) 29 Aug. 3/1: Written by a Yank ‘Down-Under’ correspondent.
1958		N.Y. Herald Trib. Book Rev. 6 Apr. 1: Then in ’51 the trek started Down Under as gold-hungry men from California set off to dig for the precious metal in Australia [W&F].
1960		R.M. Rogers Long White Cloud 8: She was back in England, right now. She’d never really followed him Down Under.
1972		P. Fordham Inside the Und. 67: Even the celluloid strip to open Yale locks originated ‘down-under’.
1985		B. Humphries Traveller’s Tool 3: A rare glimpse of corruption Down Under through the eyes of a deeply concerned, yet eminently approachable trouble-shooter.
1999		Guardian Travel 21 Aug. 6: I need my regular fix of Sydney and go Down Under almost every year.
2002		D. Robson ‘Messing about on the river’ BBC Sport 🌐 Well blow me if the jug-lugs Kiwi with the military stride hasn’t come up trumps down under.
2009		Intelligent Life Spring 76/2: He has spent barely any time Down Under.
2010		Eve. Standard 4 May 56: [headline] Wonder from Down Under.
2017		G. Gilmore Headland [ebook] ‘I never really thought about ’em when I was ... before I came down under’.

2. (Aus.) the United Kingdom; also used adverbially; see cit. 1900 for adj. use.

1900		Bulletin (Sydney) 15 Sept. 24/3: Australians take a lot of getting used to ‘down under’ climate.
1902		Bulletin (Sydney) 5 July 26/4: Swimmer George Read easily won his first race ‘down-under,’ a half-mile event at Surbiton.

3. (also way down under) New Zealand.

1916		‘Anzac’ On the Anzac Trail 2: I saw a notice in the papers calling on all New Zealanders [...] to be trained for service with the ‘Down Under’ contingents.
1945		J. Henderson Gunner Inglorious (1974) 153: For we are the boys from Way Down Under, / SONS OF THE ANZACS ARE WE!
1988		McGill Dict. of Kiwi Sl. 39/1: Down Under New Zealand and Australia, as perceived from the Northern Hemisphere.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

4. (US black) the Southern states.

1944

D. Burley N.Y. Amsterdam News 8 Apr. 8B: The town would have been better off if they had kept him in that ‘Murder-Sate’, Mississippi, Down-under.