Green’s Dictionary of Slang

coiny adj.

rich; profitable.

1891		G. Allen Dumaresq’s Daughter 54: He paints for amusement merely. He seems to be coiny. Do you know anything of him?
1903		Blackwood’s Mag. Sept. 312/1: By George! A fellow must be a coiny bird to bank with the Bank of England .
1907		Sun (NY) 24 Feb. 8/3: The year before last was meant to be a coiny one for me.
1912		E. Pugh City Of The World 237: And then there’s fellows like yourself – not too coiny – who’ve got a bit of symperfy for a poor lad.

In compounds

coiny cove (n.) [cove n. (1)]

a rich man.

1903		Blackwood’s Edinburgh Mag. 239 311/2: I had no idea you were such a coiny cove!
1911		W.R. Paterson Old Dance Master 99: Got engaged. Found my ’ands in the till one day. Guvnor a coiny cove. Well, I wanted to give the gal a treat.
1941		Baker Popular Dict. Aus. Sl.