Green’s Dictionary of Slang

1. crazy, insane, eccentric.

1974		(con. late 1940s) E. Thompson Tattoo (1977) 453: ‘You’re crazy.’ He nodded at his face in the mirror [...] ‘You’re really wack-o sometimes, man.’.
1975		J. Wambaugh Choirboys (1976) 44: If that whacko bitch wanted to take gas, fuck it, it ain’t our fault.
1986		T. Winton That Eye, The Sky 75: Maybe it’s growing up – or going whacko.
1989		Kirk & Madsen After The Ball 203: Finally, many diverse (and patently wacko) groups are permitted to publish ads.
1999		Guardian G2 9 Sept. 6: She spoke to two torn Vietnam veterans, desperate guys who had turned to wacko religion.
2002		S. Maloney Something Fishy (2006) 103: There was something a bit dangerous about the woman [...] not whacko dangerous. Something challenging and, well, sexy.
2006		C. Hiaasen Nature Girl 81: Dealey had his share of wacko clients, but Lily Shreave was breaking new ground.
2010		(con. 1973) C. Stella Johnny Porno 183: One of those whacko car buffs gets wood looking at racing magazines.
2017		S.M. Jones August Snow [ebook] ‘[Y]ou’ll just end up being dismissed as one wacko black man in a city filled to overflowing with wacko black men’.
2021		C. Hammer Opal Country 206: ‘A member of this whacko sect’.

2. confused.

2012

(con. 1980s) I. Welsh Skagboys 72: I [...] see young Grant looking wacko and the tears condensing in Maria’s eyes.