Green’s Dictionary of Slang

(US) a young woman, orig. typically from the chorus line, who swaps sexual favours for the monetary and material gifts of a (usu.) older lover.

1923		O.O. McIntyre New York Day by Day 14 Sept. [synd. col.] Even one of the shrewdest criminal lawyers New York has ever known fell under the magic sway of the diggers.
1930		W.R. Burnett Iron Man 50: She was just a plain digger, but you can’t make him believe that.
1949		Monteleone Criminal Sl. (rev. edn) 68: digger [...] a prostitute; a whore.
1960, 1975		Wentworth & Flexner DAS.
1999		Observer Rev. 1 Aug. 6: Is this a digger I see before me?