BUTTOCK and Twang,
or a down-right Buttock and sham File, a common Whore, but no Pickpocket.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Buttock and Twang in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).