Green’s Dictionary of Slang

1. an admirable person or object; an outstanding example; also as term of address .

1789		G. Parker Life’s Painter 150: Rum one. Meaning a good one.
1793		Bacchanalian Mag. 43: [of a fighting dog] I ne’re chuse to go alone / Whene’er a rum one’s baited.
1819		P. Egan Key to the Picture of the Fancy going to a Fight 16: [of bulls] [E]ven Tom Oliver’s rum one, will not bear comparison with the above prime article.
c.1825		T. Jones ‘The True Bottom’d Boxer’ in Egan Bk of Sports (1832) 74/1: You’ll find him a rum ’un, try on if you can.
1829		‘Stage-Coachman’s Lament’ in C. Hindley James Catnach (1878) 200: Ah! warn’t she a rum’un to go!
1837–8		‘Sam Swipes’ Cuckold’s Nest 19: My Sal is a rum one, as ever you see, / And chaps at her tail she has two or three.
1838		C. Selby Jacques Strop II ii: mrs g.: Do you speak Greek and Latin, Chevalier? ber.: Oh, yes [...] Like a rummun!
1842		Wkly Rake (NY) 18 June n.p.: Your father’s a rum ’un.
1846		Swell’s Night Guide 60: Ratherish, my rum’un.
1852		Kendal Mercury 17 Apr. 6/1: Here’s the balsam, my rum ’un, and your nibs must stole for the lush.
1854		Liverpool Mail 5 Aug. 5/6: The Australian rat, observe his white skin and pink peeers; he’s a rum un to bite.
c.1863		‘Prince of Wales’ Marriage’ in C. Hindley Curiosities of Street Lit. (1871) 68: Be a rum ’un to look at, but a good ’un to go.
1868		‘A Conversation on the Coming Elections’ in C. Hindley Curiosities of Street Lit. (1871) 80: Said Gladstone, Dizzy my rum ’un.
1876		C. Hindley Life and Adventures of a Cheap Jack 108: He struck me on the nose a rum ’un.
1878		Bismarck (ND) Weekly Trib. 24 May 2/4: I say, Jack, wouldn’t it be a rum ’un, ef some millionaire u’d start us up in the newspaper line?
1885		Illus. London News 7 Feb. 3/1: ‘Nature’ [...] ‘is a rum ’un’ insomuch that she enables human beings to live [...] under such circumstances.

2. a telling blow.

1824		Pierce Egan’s Life in London 24 Oct. 308/3: Bob napped a rum one on his body, which him twist .
1835		Salisbury & Winchester Jrnl 15 June 2/5: Barney goes up to kick up a shindy, and gits a rum ’un for himself.
1842		Whip & Satirist of NY & Brooklyn (NY) 1 Jan. n.p.: Curtis gave Warrne a rum-one in the middle of his head.
1848		Bell’s Life in Sydney 18 Mar. 1/4: Ben caught a rum-un in the chops.
1853		Bell’s Life in Sydney 31 Dec. 3/3: Mrs Robinson catches Charley over the back with an iron-bark stick and, myword, it was a rum ’un.

3. anything or anyone considered odd or eccentric, whether animate, inanimate or theoretical.

1789		G. Parker ‘Masqueraders’ in Farmer Musa Pedestris (1896) 72: Ye flats, sharps, and rum ones, who make up this pother.
1818		‘Thomas Brown’ Fudge Family in Paris Letter III 27: The Mounseers are but rum ones.
1821		‘An Amateur’ Real Life in London II 102: Ven you fell overboard the other day you roared like a rum un, and ven I pulled you out you squeaked like a pig, so that are proves vhat you have got two woices.
1825		‘Every Man to His Trade’ in Universal Songster I 5: I’z a countryman, just come to town / And a rum one as e’er come before you.
1831		Satirist (London) 19 June 85/2: Ow’s Sal? Putty bobbish agin? My hies! here’s a rum un a-coming!
1832		Egan Bk of Sports 3: The late George Simcock, as the term goes now-a-days, was a ‘rum one to look at.’.
1840		R. Barham ‘Smuggler’s Leap’ Ingoldsby Legends (1842) 164: [of a horse] He’s a rum one to look at – a devil to go!
1841		‘Three Yards A Penny’ Dublin Comic Songster 7: I say my rum un who are you? / What a dreadful shocking bad hat.
1850		F.E. Smedley Frank Fairlegh (1878) 57: The new pupil’s arrived, and ain’t he a rum un, jest?
1861		M.E. Braddon Trail of the Serpent 190: Well, you’re a rum ’un.
1863		Criminal Life (NY) 19 Dec. n.p.: Dover Mary and Clipper Hawkes are ‘rum ’uns’.
1875		‘Aliph Cheem’ Lays of Ind (1905) 14: ‘You air indeed a cusséd rum ’un’.
1877		Five Years’ Penal Servitude 168: Warning me that I had two ‘rum ’uns’ alongside of me, he left me to go to someone else.
1880		Bulletin (Sydney) 11 Dec. 9/4: Bob Wilson is a ‘rum ’un’. His latest development is a sort of cross between Exeter Hall and [...] the Rev. Peter Campbell.
1880		E.J. Milliken ‘Cad’s Calendar’ Punch Almanack n.p.: Fan gets married ; ah ! them gurls is rum ’uns !
1894		G. du Maurier Trilby 72: He’s a rum ’un, ain’t he?
1907		D. Stewart Shadows of the Night in Illus. Police News 31 Aug. 12/3: He’s a goin’ to the Waterman’s Arms [...] There’s a few rum ’uns goes to that crib’.
1910		H.G. Wells Hist. of Mr Polly (1946) 100: You’re a rum un.
1926		K.S. Prichard Working Bullocks 285: A rum ’un to look at but a beggar to go.
1932		M. Marshall Travels of Tramp-Royal 236: Curse me if you ain’t a rum ’un, Scottie, and a reg’lar dark hoss.
1954		Oh Boy! No. 17 4: That prisoner is a rum one!