Green’s Dictionary of Slang

1. (20C+ mainly Aus., also gray) a halfpenny or other coin, having two heads or two tails, esp. as used in cheating games .

1812		Vaux Vocab. of the Flash Lang.
1828		G. Smeeton Doings in London 40: Breslaw could never have done more upon cards than he could do with a pair of ‘grays’ (gaffing-coins).
1831		Satirist (London) 20 Nov. 262/1: Breslaw could never have done more upon cards than he can do with a pair of ‘greys’ (gaffing coins).
1845		Launceston Examiner (Tas.) 29 Jan. 3/4: Mr. King was suspected of [...] using a halfpenny with the Britannia impression on both sides [...] It appears this species of fraud is not uncommon, and [is] understood by the technical slang of ‘playing the grey’.
1855		Bell’s Life in Sydney 3 Mar. 2/7: A certain article denominated a ‘grey’, id est, a halfpenny with ‘tail’ on both sides.
1859		Hotten Dict. of Modern Sl. etc. 47: grays, half-pennies, with either two ‘heads’ or two ‘tails,’ — both sides alike. Low gamblers use grays, and they cost from 2d. to 6d. each.
1868		‘Six Years in the Prisons of England’ in Temple Bar Mag. Nov. 539: The way they do it is to have a penny with two heads or two tails on it, which they call a ‘grey,’ and of course they can easily dupe flats from the country. [...] I suppose they have named it after Sir George Grey because he was a two-faced bloke?
1870		Hotten Sl. Dict. [as cit. 1859].
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: A ‘gray’ is a half penny with two heads or two tails, and is supposed to be a delicate compliment to nobleman who interested himself in the suppression of swindling and gambling.
1889		Dead Bird (Sydney) 26 Oct. 3/1: At four I learned to work a ‘grey’.
1894		Truth (Sydney) 24 June 1/4: He skied a ‘double-headed grey’, / And turned a politician.
1898		W.T. Goodge ‘Australia’s Pride’ in Bulletin 3 Sept. 32: He’d simply smashed the two-up school – / [Assisted by a ‘grey!’].
1900		Bulletin (Sydney) 13 Oct. 14/3: [T]hough the gentle ‘two-up’ has been played in the yards of these pubs. regularly every Sunday for ages past, only one arrest has been made within the memory of man – when a big bettor was found using a ‘grey’.
1904		Sun. Times (Perth) 22 May 4/8: I don’t spin a nob or a grey.
1904		Sydney Sportsman (Surry Hills, NSW) 2 Mar. 8/5: He alleged that one of Suqui's coins was a ‘gray’. As some innocent readers of the ‘Sportsman’ may not know what a ‘gray’ is, it may be explained that it is a coin with two heads, no tail.
1948		R. Raven-Hart Canoe in Aus. 187: Pennies supplied by ‘school’ to avoid ‘nobs’, ‘jacks’, double-sided heads, ‘greys’, double-sided tails.
1999		G. Seal Lingo 147: A grey was a two-headed or two-tailed coin, used in various games, though nowadays best known in two-up.

2. money in general [the colour of ‘silver’ coins once they have been some time in circulation].

1909

J. Ware Passing Eng. of the Victorian Era 147/2: Grey (Thieves’). Evasive name for silver – from its colour presumably; and figuratively, money.

3. (N.Z. prison, also greycoat, grey day) a 100mg morphine sulphate tablet [the tablet is grey].

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 81/2: grey(also greycoat) n. a 100mg morphine sulphate tablet [...] grey day n. a 100mg morphine sulphate tablet .

4. see gray n.