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Knight of the rainbow; a footman: from being commonly clothed in garments of different colours. A meeting of gentlemen, styled of the most ancient order of the rainbow, was advertised to be held at the Foppington’s Head, Moorfields.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Rails * Rainy DayNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes
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Francis Grose was independently wealthy, having inherited money from his father, a jeweller. Finding himself overspending, he published a number of books; his Provincial Glossary seems to have been the starting-point for the Vulgar Tongue reproduced here.