Green’s Dictionary of Slang

(orig. US) good luck, esp. in quantity and unexpected.

[1876		‘Dan de Quille’ Big Bonanza (1947) 115: There are always some companies in ‘borrasca’ – out of luck, in barren rock – while others are in ‘bonanza’ – in good luck, working large bodies of rich ore].
1879		R. Taylor Destruction and Reconstruction 50: If silence be golden, he was a ‘bonanza’ .
1887		‘Lawrence Lynch’ Mountain Mystery 34: ‘Well, I swan!’ ejaculated Podunk. ‘She’d be a bonanza to some man who wanted a wife.’.
1899		A.C. Gunter M.S. Bradford Special 72: Thirty dollars a week is a bonanza to a stenographer.
1911		G. Bronson-Howard Enemy to Society 160: Unaccompanied gentlemen in fur-lined coats and evening dress had been bonanzas to ‘The Gem’ before.
1926		Wood & Goddard Dict. Amer. Sl.
1934		(con. 1880s) E. Cunningham Triggernometry (1957) 241: Boys, we’ve hit the damn’ bonanza!
1962		(con. late 19C) E. Lucia Klondike Kate 67: The early tinhorns and slickers hadn’t realized yet what a giant bonanza they were sitting on.
1978		P. Theroux Picture Palace 213: The more Orlando kidded me about the bonanza at the Camera Club, and all those requests for photojournalism.
1990		Eble Campus Sl. Nov. 1: bargain – stroke of great luck [...] Also bonanza, bargain-bonanza.
1999		K. Sampson Powder 360: James worked his fingers inside his hip pocket, knowing there’d be a bonanza, and was not disappointed.