Green’s Dictionary of Slang

a division of the spoils.

1883		Nat. Police Gaz. (NY) 10 Nov. 6/1: [headline] A Whack-Up With Officer Coghlan.
1896		Ade Artie (1963) 63: He hadn’t been in on the whack-up six weeks.
1911		J. London ‘Burning Daylight’ in Atlanta Constitution 2 Apr. 2/5: That-all’s taken from the winnings before we-all get to figurin’ on the whack-up.
1912		Ade Knocking the Neighbors 210: When it came time to Scramble the Eggs and pull off the grand Whack-Up, he was standing at the head of the Line.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxiii 4/3: chop: Share of loot, see also corner. ‘How many in the ‘chop-up’, also ‘carve up, whack up.’.
1973		J. McNeil The Chocolate Frog and The Old Familiar Juice 115: .
1990		R.G. Barratt ‘Kill Two Birds’ in What Do You Reckon (1997) [ebook] They cop half the $20 million. The rest is a two-way whack-up between the State and Federal Governments.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 74: ‘The the day after the whack-up someone not entirely unlike [...] Waters arrives on the scene’.
2001		(con. 1945–6) P. Doyle Devil’s Jump (2008) 98: The coupons are in the car. We can do a whack up right now.