FLUSH in the Pocket,
full of Money. The Cull is Flush in the Fob, the Spark's Pocket is well lin'd with Money.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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Flush In the Pocket in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).