1736 Canting Dictionary / F / FLAG [money,adj]
a Groat; The Flag of Defiance is out, (among the Tarrs) the Fellow's Face is very red, and he is drunk.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Flag in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).