Green’s Dictionary of Slang

whoaball n.

also who-bawl, whow-ball, woball
[SE whoa, stop + Ball, a common name for a cow]

a milkmaid.

[UK]B.E. Dict. Canting Crew n.p.: Whow-ball, a Milk-maid.
[UK]N. Ward London Spy XIII 308: [She] was as Angry when we spoke to her, as a Milk-Maid when you cry Wo-ball.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Whow-ball, a milkmaid, from their frequent use of the word whow, to make the cow stand still in milking. Ball is the supposed name of the cow.
[UK]G. Andrewes Dict. Sl. and Cant n.p.: Wo ball a milk woman.
[UK]Lex. Balatronicum [as cit. 1785].
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 194: Who-bawl — a milk-woman, calling to her cows; and, by refraction, she herself.
[UK]G. Kent Modern Flash Dict. [as cit. 1809].
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]Duncombe New and Improved Flash Dict.
[US]Matsell Vocabulum 96: woball A milkman.