Green’s Dictionary of Slang

1. a crazy, eccentric, or extreme person.

1938		Nevada State Jrnl 11 June 6/4: In this New York area we have a mythical but powerful organization known as the ‘Wackoes.’ It includes most of the boxing writers and certain celebrities connected with the punch profession. The test of power in this organization is one's reputation for being a ‘screw-ball’ at any and all times.
1960		J. Walker No Sunlight Singing (1966) 16: Tommy get the metho; Anni get the whacko; heh! heh!
1972		B. Rodgers Queens’ Vernacular 208: wack-o (fr colloq wacky = erratic) irrational human being.
1975		J. Wambaugh Choirboys (1976) 79: ‘That prick Lieutenant Finque is doing it to me.’ [...] ‘Roscoe or one a those whackos’ll think up some way to fix his ass.’.
1981		H. Gould Fort Apache, The Bronx 330: The guy’s a whacko, Murf.
1994		P. Quigley Borderland 167: Messages had been scrawled between the metal ridges – ‘Wacko loves Tina.’.
2000		Guardian G2 18 Jan. 17: A subcultural freakshow of fringe artists and other assorted wackos.
2005		Mad mag. July 18: The man’s a gibbering, paranoid wackdoodle.
2011		J.J. DeCeglie Drawing Dead [ebook] I [...] had to go to see this wacko because I badly needed the coin.
2016		New Yorker 11-18 July 🌐 So, yes, there are bigots in the Trump movement, and wackos, and dummies.
2021		C. Hammer Opal Country 148: ‘How was the Seer?’ ‘A whacko [...] crapping on about reading auras’.

2. an unstable or mentally ill person.

1960		Mad mag. Sept. 48: Sammy Whackoe flew to Frisco last week – swears he’ll take a plane next time.
1977		H. Ellison Flop Sweat in Shatterday (1982) 63: A certifiable nut-case, a card-carrying whacko.
1985		(con. 1968) D.A. Dye Citadel (1989) 270: Maybe someone should certify once and for all which ones are the real whackos.
1986		R. Campbell In La-La Land We Trust (1999) 65: She’s smelled [fear] on him when those two wackos were kicking the head around.
1996		P. Cornwell Cause of Death (1997) 325: Maybe you can explain to them the importance of the bible these wackos believe.
1998		J. Hoskison Inside 167: The £120,000 given to the whackos in the Stress Clinic was presumably not [too expensive].
2004		J. Ellroy ‘Jungletown Jihad’ in Destination: Morgue! (2004) 366: I’m considered a wack within LAPD.
2005		P. Howard Curious Incident of the Dog in the Nightdress 89: I wanted her to see what kind of a focking wacko you really are.
2020		D. Winslow ‘The San Diego Zoo’ in Broken 126: A] wacko, a sick headcase [...] who wanted to see what would happen if you gave a handgun to a chimp.