Green’s Dictionary of Slang

to make a great deal of money.

1856		C. Reade It Is Never Too Late to Mend II 294: I see all this money-making is a contrivance to keep me in Sydney. It is absurd my coining paint at this rate.
1887		in G.D. Atkin House Scraps 126: The Bulls are coining money.
1899		J.M. Harrell Hot Springs Doctor 70: Torrence was then ‘coining money’.
1905		‘Hugh McHugh’ You Can Search Me 48: We can simply coin money with him.
1912		Bulletin (Sydney) 14 Nov. 12/3: Why, with vegetables at the present prices, he’s coining money!
1925		Wodehouse Carry on, Jeeves 226: ‘Twenty-five pounds.’ ‘Good lord, Jeeves! You’ve been coining the stuff!’.
1937		D. Fuchs Low Company 209: Hey, this joint certainly coins the dough.
1941		K. Tennant Battlers 21: You’d be a knockout outside the pubs on a Saturday afternoon. [...] We’d coin money.
1959		F. Norman Fings I i: I do me fair stint / I’m coinin’ a fair mint.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 88: I’ll bet you’re coining it in.
1989		T. Blacker Fixx 112: We were bloody coining it.
1998		I. Welsh Filth 62: That lippy fucker Conrad Donaldson Q.C. who spends his time coining it from the tax-payer.
2005		P. Howard Curious Incident of the Dog in the Nightdress 20: He’s coining it in selling those gaffs in Camolin.