(informal) Exceptionally large; whopping (often followed by an adjective such as great or big).
1762, Thomas Bridges, A Burlesque Translation of Homer, London: S. Hooper, 1772, Book 7 of Homer’s Iliad, p. 289,
[…] all our grannies tell us how
He kill’d a whacking great dun cow;
1819, Olivia Clarke, The Irishwoman. A Comedy in Five Acts, London: H. Colburn, Act V, Scene 2, p. 80,
[…] these two whacking Irish boys, that I was telling you of just now, are posted at the hall door to seize the villain, and take him to pay his respects to the next sitting magistrate […]
noun
(countable) A beating.
(uncountable) Alternative form of waacking (“dance style”).