Green’s Dictionary of Slang

1. (US) an eccentric.

1907		N.Y. Eve. Journal 7 Feb. n.p.: If you didn’t have a bunch of grogans under your chin your were a rum.
1913		‘Lord Ballyrot in Slangland’ in Tacoma Times (WA) 5 Aug. 4/4: He makes Gainsborough and Landseer and the the rest of those rums look like beer sign artists.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 213: rum, n. – someone who is obtuse or doesn’t conduct himself in a conventional manner.

2. (US black) a fool, a dupe, a victim.

1902		T.A. Dorgan in Zwilling TAD Lex. (1993) 70: The ‘rum’ who continually laughs and occasionally swallows the ball.
1907		B. Fisher A. Mutt in Blackbeard Compilation (1977) 3: I’m a rum for working for $10 a week. I see here where a guy copped a million on the track.
1915		Van Loan ‘One-Thirty-Three – Ringside’ in Taking the Count 73: Serves ’em right for going around the country trying to make a rum out of me!
1916		J. Lait ‘Canada Kid’ in Beef, Iron and Wine (1917) 153: I takes a rum wit’ a plug hat an’ a frock-coat what’s walkin’ wit’ a umbrella in his duke an’ a cigar in his face.
1941		D. Burley N.Y. Amsterdam Star-News 22 Nov. 14: A square ain’t nothing but a Lane and a Lane ain’t nothing but a Rum and a Rum ain’t nothing but a Perfect Lamb.
1957		D. Burley N.Y. Amsterdam News 8 June 14: A Perfect Rum with plenty of bread who is really dumb .
1969		B. Jackson Thief’s Primer 143: Down here in prison, one of these rums, one of these idiots, he’s not going to get in my face.
1976		M. Braly False Starts 42: I quickly learned this tunic was considered hopelessly square and worn only by fish and rums.

3. a drunkard.

1909		Eve. Star (Wash., DC) 11 Sept. 20/2: I’ve often heard the rums talking to themselves [...] ‘Me t’ get off th’ stuff, quick’ I heard one of the souses muttering.
1960		C. Cooper Jr Scene (1996) 104: If that rum is still outside, tell him I said to get the hell away from here.