Green’s Dictionary of Slang

(Aus.) a term of the highest praise, something excellent.

1904		Bulletin (Sydney) 14 Apr. 29/1: A bontosher is a real slasher, a fair hummer, virtually a past master [...] but no female has yet achieved the dignity of a bontosherina. [Ibid.] 5 May 17/3: ‘Bonster’ is a corruption of Bontojer, pronounced Bontodger, and Bontojer is a corruption of the two French words bon and toujours — ‘always good’.
1904		Sydney Sportsman (Surry Hills, NSW) 14 Sept. 6/1: I can promise my readers a bon-todger battle.
1907		E.Dyson in Bulletin (Sydney) 18 July 39/2: ‘Better’n a pantermine, he sez. Sez he never larfed so much since mother blew up.’ ‘That’s bon tosh,’ said Cilly, with enthusiasm.
1911		E. Dyson ‘Barracking’ in Benno and Some of the Push 144: ‘Oh, a bonzer, a bonzer, a boshter, a bontoshter!’ screamed our Christian brother.
1913		Sydney Sportsman 15 Jan. 2/6: Now while we credit Tom with being a bon-tosher on the cloth of green, still we watched every move of our newly found friend Cecil.
1914		Sun. Times (Perth) 11 Jan. 3/2: Tuesday evening saw a duststorm – a beauty, a regular bontosher – and that gave us a cool night and brought temperatures down.
1924		Wkly Judge (Kalgoorlie) 22 Feb. 2/3: With a bontosher like Claro in the Newmarket, the Victorian trainers have something warm to beat.
1932		Advocate (Burnie) 11 Mar. 8/5: It is no uncommon thing for an enthusiastic skip [...] to coin fresh phrases about particular bowls that are making advance to ‘kitty,’ and such terms as ‘bobby dazzler,’‘bontosher’ and right on the doodah’ are often heard.
1945		Baker Aus. Lang.
1951		D. Stivens Jimmy Brockett 141: It was a bontosher of a flat though. I’d slipped in during the afternoon to have a decker at it.
1999		(ref. to 1900s) G. Seal Lingo 58: bon had been in Lingo usage for more than a decade before World War I began in such combinations as it’s a bon, ain’t it [...] bonsterina, bon ton [...] and bontosher.