Green’s Dictionary of Slang

1. a pipe, presumably long-stemmed.

1830

Egan Finish to the Adventures of Tom and Jerry (1889) 76: I shall bring down my ‘long one,’ which will enable me to add a few more clouds to your neighbourhood.

2. (Aus.) a tall glass of beer.

1880		Bulletin (Sydney) 6 Nov. 8/2: Mrs. Fredericks, the charming wife of the charming tenor, has. [...] taken the Carlisle Castle Hotel, in Newtown. There is a rumour [...] that when a lucky mortal partakes of a ‘long ’un’ of her drawing, he goes about all day with a halo round his head.
1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 119: Here’s a long one for you; make the most of it.
1892		H. Lawson ‘Constable M’Carty’s Investigations’ in Roderick (1967–9) I 195: Mac had noticed that the lodger did a mighty lot of smoking / And could ‘stow away a long ’un’ never winking, so he could.
1902		Ade Forty Modern Fables 201: Elvira had to brace herself and hold on to something when she got rid of a Long One.
1905		Sun. Times (Perth) 26 Nov. 4/7: Y is the yearning a ‘long ’un’ to sink.
1930		N. Lindsay Redheap (1965) 48: ‘He’s onter me for boozin’. Sees me comin’ out o’ Cassidy’s full as a tick. Eighteen long uns I had’ .
1937		H. Drake-Brockman Blister Act I: ’Struth, it’s hot. I want a long ’un.

3. (US) a horse that is listed at long odds, an outsider [long adj. (1)].

1899–1900		C.L. Cullen Tales of the Ex-Tanks 365: If one of ’em [i.e. a bettor] gets wise to a long one that’s due to be uncorked, it’s handed around to ’em all.
1971		L. Sanders Pleasures of Helen 65: ‘About a month ago I hit a long one for almost a hundred’.

4. (UK Und.) a long prison sentence.

1970		G.F. Newman Sir, You Bastard 101: Roffey and Chrostie would need to be protected by some divine force not to draw long ones.
1995		J. Cameron Vinnie Got Blown Away 69: You don’t even know you get a guilty, let alone you get a squeeze or a long one.

5. see long ’un under long adj.