A milk-maid: from their frequent use of the word whow, to make the cow stand still in milking. Ball is the supposed name of the cow.
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.
Whore Pipe * WibbleNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes