MUSICK,
the Watch-word among High-way-men, to let the Company they were to rob, alone, in return to some Courtesy from some Gentleman among them.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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The Vulgar Tongue originally collected by Grose, 1811.
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).