Green’s Dictionary of Slang

1. (a glass of) gin [note cit. 1811].

1811		Lex. Balatronicum n.p.: Drain. Gin: so called from the diuretic qualities imputed to that liquor.
1819		P. Egan Key to the Picture of the Fancy going to a Fight 19: Mr Wastebut [...] has got the max in his hand, ready to supply [...] a dashing drain.
1826		Pierce Egan’s Life in London 17 Sept. 683/1: I had a drain to stifle the blues.
1838		‘Paul Pry’ Oddities of London Life I 24: [W]e met Pat Welch, and so we stopped to have a drain and a spache about Ireland.
1842		‘The Saint Giles’s Jade’ in Nancy Dawson’s Cabinet of Songs 45: All max from her belly he barr’d, / Not a drain could she get ’pon my vord.
1848		Sam Sly 9 Dec. 4/2: The British Sailor was cursing Britannia for not lending him twopence for another drain.
1859		G.A. Sala Gaslight and Daylight 263: The Holborn gin-shop habitués [...] the regular dram-drinker, who takes his ‘drain’ and is off.
1865		T. Archer Pauper, Thief and Convict 127: Old Peg could tell you some queer stories if you could get hold of her and stand a drain, and give her a bit of tobacco.

2. a drink; thus do/take a drain, to have a drink (with a friend).

1821		W.T. Moncrieff Tom and Jerry III iii: Now, landlord, arter that ’ere drap of max, suppose we haves a drain o’ heavy wet, just by way of cooling our chaffers – mine’s as dry as a chip.
1838		Dickens Oliver Twist (1966) 209: ‘A drain for the boy,’ said Toby, half-filling a wine glass.
1846		Swell’s Night Guide 61: Daddle your fam, Billy, its all square. Come, Sall, speel, come hook it; Billy’s good for the drainums. [Ibid.] 66: Vell, culley I’m jest stalling over to the lush crib to a pal, for a drain of hevy.
1849		Paul Pry 15 Jan. n.p.: R—d F—lf—d, alias [...] alias the Clare market ‘Adonis’ [...] not to be seen so frequently ‘doing a drain’ with the ‘Pollys’ in Fleet street.
1853		W. Howitt Two Years in Victoria (1855) I 344: ‘Have you got a drain, then?’ (grog).
1854		G.J. Whyte-Melville General Bounce (1891) 323: Take another drain, Mr Fibbes: talking’s dry work.
1855		Bell’s Life in Sydney 7 July 3/2: It really seems as if many of those nobblerizers considered the Benevolent Asylum as a sort of assurance association [...] As they are so fond of ‘drains’, would it not be a good plan to oblige them to labor [...] on Mr. Randle's sewer works?
1857		Argus (Melbourne) 6 Jan. 6/3: ‘Modern Fast Conversation’ [...] He does not drink beer, but ‘does malt,’ or (the nasty follow) ‘takes a drain’.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 133: DRAIN, a drink; ‘To do a drain,’ to take a friendly drink.
1866		J. Greenwood Little Ragamuffin 35: We shan’t be able to get a drain at the ‘Stile’ before they shuts up.
1877		Five Years’ Penal Servitude 238: First they went to the bar and had a drain.
1883		R.L. Stevenson Treasure Island 20: If I don’t have a drain o’ rum, Jim, I’ll have the horrors.
1895		Crowe Aus. Sl. Dict. 25: Drain, a drink, as ‘to do a drain,’ to have a drink.
1905		‘The Old Keg of Rum’ in ‘Banjo’ Paterson Old Bush Songs 91: For to give another drain / The old keg would refuse.
a.1905		Murray, Leigh & LeBrunn [perf. Marie Lloyd] Sleep! Sleep! 🎵 [H]e goes and has a drain; / Comes in from the pub, eats up all the grub.
1918		C.J. Dennis ‘Doch-An-Doris’ in Backblock Ballads 18: But rain or shine have one more drain! / The last — the Doch-an-doris!
1928		D. Lawley Hustling Hobo 253: Better come and have a drain if yer wants to.
1933		‘William Juniper’ True Drunkard’s Delight.
1958		J. Arden Live Like Pigs Act IV: Give us a drain for her, Col... (He seizes a bottle from Col, and gives it to Daffodil.).
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 34: Nothing much can happen while we have a few swift drains.