Green’s Dictionary of Slang

nutso n.

[nuts adj. (2)]

(orig. US) a stupid, unstable or eccentric person; also as adj.

‘Archie’ strip in Las Cruces Sun-News (NM) 25 Apr. 11/5: Who are ‘Sonny’ and ‘Nutso’? Two folk singers?
[US]Clovis News-Jrnl (NM) 19 Mar. 6/3: Dear Betty. Don’t you think the girls who have nothing better to do than squawk about women’s rights are Nutsos?
P. Loraine Ask the Rattlesnake 12: He was probably just a nutso [...] ‘That’s it,’ she said [...] ‘He’s nuts.’ .
[NZ]K. Dunn Geek Love 336: The nutso wants to sell me a nine-millimetre full-auto with a clip as long as an elephant’s dong.
[UK]Guardian Guide 12–18 June 19: A nutso cousin in flight from her disapproving parents.
[US]M. Miner ‘Kidnapped’ in C. Rhatigan and N. Bird (eds) Pulp Ink 2 [ebook] She had those nutso blue eyes, and that laugh.