Green’s Dictionary of Slang

second-rate, tawdry, counterfeit; also as n.

1902		Truth (Sydney) 15 June 8/3: Dressed in flash-cut clobber, loud, sir, / Brummy rings & sich like show.
1910		Worker (Wagga, NSW) 25 May 3/5: ‘Where did you git that stud?’ [...] ‘It’s a brummy’.
1914		Bulletin (Sydney) 10 Sept. 48/2: Has someone used his talents in making ‘brummy’ quids, / Instead of rightly robbing the widow and her kids?
1924		C.J. Dennis ‘Nocturne’ in Rose of Spadgers 55: I knoo the sort of gold – / Priced in the brumy [sic] shops four an’ a zac.
1930		K.S. Prichard Haxby’s Circus 117: The big brummy diamond ring she was so fond of.
1941		A. Gurney Bluey & Curley 18 Mar. [synd. cartoon strip] I’m just makin’ sure I didn’t get back the brummy two bob piece I started with.