Green’s Dictionary of Slang

extremely cold in temperature; often as brass monkey weather.

1890		Sporting Times 29 Mar. 2/1: Owing to the mildness of the weather many ‘brass monkey’ nights have been put off.
1899		A. Binstead Gal’s Gossip 40: It is wicked, no doubt, for me [...] to find fault with the weather, but I freely confess that the present sample is a trifle too brass-monkey.
1959		F. Norman Fings I i: ‘I see you’ve still got yer old overcoat on.’ ‘Course I ’ave. Brass monkey wevver, ain’t it?’.
1963		P. Larkin letter 25 Jan. in Thwaite Sel. Letters (1992) 349: A slight thaw making me feel a little less brass-monkey-like, I pick up my pen.
1986		C. Bowles G’DAY 68: In Australia the weather varies little. [...] It is either ‘pissing dsown’ (Melbourne), ‘brass monkeys’ (Canberra) or ‘bloodyot’ (Perth).
1986		F. Taylor Auf Wiedersehen Pet Two 78: It was totally brass monkey’s out in the village’s one phone booth, where Dennis stood chilled to the marrow.
1996		P. Howard The Joy (2015) [ebook] It’s freezing. Real brass monkey weather.
1999		(con. 1979–80) A. Wheatle Brixton Rock (2004) 101: An alarm bell that chased them through the brass-monkey air.
2000		Sun 23 Mar. 38: You’d best shut th’window, then ... ’appen it’s brass monkey weather out there.
2003		M. Haddon Curious Incident of the Dog 55: It’s brass monkeys out there.