Green’s Dictionary of Slang

1. anything of excellent quality; an outstanding person or thing.

1908		Truth (Brisbane) 13 Sept. 1/8: [T]he usurping Sultan of Morocco has been captured by one of the deposed Sultan’s generals who rejoices in the name of Boshta ben Bagodi. The latter’s delighted master should now promote him to the rank of Bonserino at least.
1909		Star (Sydney) 8 July 13/3: ‘I fight many funny scraps, but scrap with dat Mister Douglas, of de Sout Africa, was de bonzerino’.
1918		Sun (Sydney) 4 July 6/3: And all the rank and file of the party cheered and cheered, and acclaimed him as a bonzerino.
1923		Barrier Miner (Broken Hill) 26 Sept. 3/7: [advert] [T]o cut a long joy story short we will tell you – it was a ‘BOOMERANG’ MOUTH ORGAN. A Bonserino!
1933		Smith’s Wkly (Sydney) 2 Dec. 8/4: I stood near a Legerite who urged him lustily. ‘Oh, you little ooty booty – your mother was a racehorse,’ he cried. ‘He’ll --- it in. Come on, you bonserino’.
1941		Australasian (Melbourne) 29 Mar. 3/1: Definitions. – Crackerjack: Greatest fighting tedder of all history. Bonzerino: Man of indomitable courage and invincible ideals.

2. good fun; revelry and merriment.

1932

Wkly TImes (Melbourne) 2 Jan. 21/3: And they bethought them that they would have a night of mirth, and musick, and bally-hoo, and general bonzerino.