FLASH,
a Peruke. Rum Flash, a long, full, high-priz'd Wig. Queer-Flash, a sorry weather-beaten Wig.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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Flash in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).