Green’s Dictionary of Slang

whack n.5

also wack
[abbr. whacko n.]

1. (US) a mad or eccentric person; occas. of a place; also as adj.

[US]H.M. Anderson Strip Tease 32: The other girls tell her to lay off this guy. He’s a wack.
[US]N. Algren Never Come Morning (1988) 117: The whack knew he was a lefthander after all.
[US]R.L. Bellem ‘Dead Don’t Dream’ in Hollywood Detective July 🌐 He was just a witty wack with a whisky weakness.
[US]S.J. Perelman letter 23 Dec. in Crowther Don’t Tread on Me (1987) 147: In short, he’s a wack, extremely good company.
[US]Mad mag. Jan.–Feb. 14: This Billy whatzisname, this Poobah whack out in Kansas.
M. Braun Love Me Do 13: ‘Your ever-hopeful Beatle-loving whack’.
[US]L. Bruce How to Talk Dirty 22: In the Army you can get out if you’re a wack.
[US]R. Price Ladies’ Man (1985) 7: There was nobody out on Broadway except bag ladies and street whacks.
[UK]A. Payne ‘You Need Hands’ Minder [TV script] 8: Who do you support, wack?
[US] P. Munro Sl. U.
[UK]Guardian Guide 8–14 Jan. 26: Man, this town is so wack!

2. (US campus) a fool.

[Aus]Penguin Bk of More Aus. Jokes 342: We call him Whacker the Australian, or Whack for short.

In compounds

whack attack (n.)

(US black) the onset of apparent insanity, usu. through the use of drugs.

[US]Bentley & Corbett Prison Sl. 51: Whack Attack Acting crazed when experiencing the effects of PCP or an hallucinogenic drug.
whack job (n.) [job n.2 (4a)]

(US) an insane person, a madman; also attrib.

[US]E. Torres After Hours 196: The real whack-jobs would get mad.
[US]R. Price Clockers 320: Let’s make sure he’s not some whack job who confesses to everything like a hobby.
[US](con. 1986) G. Pelecanos Sweet Forever 252: He wasn’t up for small talk with Wack-Job Wanda tonight.
[US]G. Pelecanos Shame the Devil 59: Vance’s friends got a big charge out of it. Vance’s dad, the Vietnam vet and mail carrier — with that combo, he had to be some kind of wack job, right?
in Times (L) 29 Oct. 19: A writer [...] quoted a ‘senior’ McCain adviser calling Mrs Palin a ‘whack job’, which roughly translates as worringly off her trolley.
[US]C. Hiaasen Star Island (2011) 82: He was actually an okay guy [...] for a whack job.
[US]S. King Finders Keepers (2016) 317: He’s crazy. A total whack-job.
[US]S.M. Jones Lives Laid Away [ebook] ‘You have any idea how many of these whack-job groups there are in Michigan?’.