An obsolete English slang word meaning "ready money." Hickeringill, in his Works, 1682 (ii. 20), says, "down with the dust and ready Darby," and Shadwel, in his play The Squire of Alsatia, 1688 (i. 1), uses the expression, "the ready, the Darby."
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)