Green’s Dictionary of Slang

John Davis, the n.

(US) money.

1872		Schele De Vere Americanisms 296: Among the less generally known terms are John Davis or the Ready John, sometimes simply John or Ready [etc.].
1905		Exeter & Plymouth Gaz. 4 Feb. 5/6: Money in general is known as: The Actual, Coliander Seeds, [...] Hard, John Davis, King’s Pictures, [...] Nonsense, Oil of Angels, [...] Rowdy.
1919		Smith’s Wkly (Sydney) 7 June 9/6: Slang of Money [...] It has been called ‘the actual, the blunt, hard, dirt, evil, flimsy, gilt, iron, John Davis, lurries, moss, oil of angels, pieces, rowdy, spondulicks, tin, wad’ .