Milk and water, with a little rum, and some nutmeg; also the name of a composition used by distillers, to make spirits appear stronger than they really are, or, in their phrase, better proof.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
To Dock * DoctorsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes