Green’s Dictionary of Slang

whack n.2

also wack
[whack v.1 ]

1. [late 18C+] a blow, usu. with some form of stick.

2. a second-rate or inexperienced coachman [his reliance on the whip].

3. [1940s] (US Und.) a prison sentence.

4. [1960s+] (Scot./Aus.) a punishment, one’s deserts.

5. [1980s+] (drugs) the act of diluting bulk drugs, e.g. heroin, for retail sale.

6. skill, ability.

In phrases

black whack (n.)

[1980s+] (drugs) phencyclidine.

cop one’s whack (v.)

[1970s+] (Scot./Aus.) to get one’s deserts.

go to whack (v.)

[1910s+] to collapse, to break down (lit. and fig.).

out of whack

[late 19C+] lit. or fig. off-centre, out of true, out of order, feeling unwell.