BLACK-GUARD,
dirty, nasty, tatterred roguish Boys, that formerly were wont to attend at the Horse-Guards to wipe Shoes, and clean Boots.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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Black Guard in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).