Green’s Dictionary of Slang

money, spec. a dollar (see cite 1936).

1885		Bulletin (Sydney) 1 Aug. 14/2: The Deniliquin Jockey Club Midwinter meeting last week proved a great success, both as regards weather, attendance, and the raking in of the ‘doubloons’ to the club’s coffers.
1906		Times Dispatch (Richmond, VA) 7 Oct. 48/2: Your head would swim if I told you the number of doubloons they spend in a year.
1909		J.W. Carr in ‘Word-List From Northwest Arkansas’ in DN III:v 395: doubloons, the, n. Money.
1913		Day Book (Chicago) 24 May 23: I needed the doubloons.
1919		Wodehouse Damsel in Distress (1961) 198: ‘You mean has the mater the first call on the family doubloons?’ said Reggie. ‘Oh, absolutely not! [...] She has her own little collection of pieces of eight, and I have mine.’.
1926		H.C. Witwer Roughly Speaking 14: Tommy Brown had told the world about her doubloons.
1930		E. Waugh Vile Bodies 128: As soon as the governor kicks the bucket, and I come in for the family doubloons and pieces of eight.
1936		D. Burley in Chicago Defender 7 Mar. 11: She was nicked for a bit more than 600 doubloons.