MUFF,
a Woman's Secrets, To the well wearing of your Muff, Mort; To the happy Consummation of your Marriage, Madam. A Health.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Muff in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).