Green’s Dictionary of Slang

1. (orig. US) to divide (loot).

1887		Lantern (New Orleans, LA) 15 Sept. 4: Alderman Carey did whack up with brother members on this fake contract.
1891		Electrical Rev. (Century) n.p.: The city has never whacked up with the gas company [F&H].
1901		J. Flynt World of Graft 94: One of us used to go direct to the percentage coppers on the force, tell ’em our tale o’ woe, whack up the plunder, an’ stop worrying.
1906		A.H. Lewis Confessions of a Detective 17: The dip, when he trims a sucker, whacks up with the barman.
c.1910		‘O. Henry’ letter in Rolling Stones (1913) 291: If he uses mine, we’ll whack up shares on the proceeds.
1934		H. Miller Tropic of Cancer (1963) 312: We emptied our pockets quickly and began to whack it up [...] It was bewildering. There was French, American and English money.
1948		Z.N. Hurston Seraph on the Suwanee (1995) 890: We’ll whack up the lunch. You and Janie take all you want.
1955		‘Blackie’ Audett Rap Sheet 184: There was quite a lot still left in the storeroom in the laundry. So the next morning we whacked this up with the rest of the boys.
1963		Sacramento Bee (CA) 11 Aug. 26/3: The difference between a gambler who whacks up (divides his winnings) and one who whacks out (goes broke).
1971		J.D. Horan Blue Messiah 309: He gives her a hundred bucks to whack up among them.
1979		E. Torres After Hours 52: Walberto glommed all the money. Probably whacked it up with your partner.
1989		H. Rawson Dict. of Invective (1991) 409: The idea of craziness may also be reinforced by out of whack, meaning ‘out of order’, which in turn derives from whack, meaning a share or portion of a larger amount (to whack up = to divide).
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 71: ‘Just whack it [i.e. stolen money] up three ways, even’.
2005		M.B. ‘Chopper’ Read Chopper 4 227: The very best of friends will gather to whack up the booty.

2. (US drugs) to dilute drugs by adding a whack n.² (5), thus increasing the weight and potential profits.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 37: I take an eighth and I whack up two ounces [56 grams] of it, and sell the whacked stuff by the gram, half-gram, like that.
1992		R. Price Clockers 401: He dropped the package on the empty seat [...] ‘Rodney say whack it up.’.

3. see whack v.² (1)