Green’s Dictionary of Slang

1. a heavy drinker.

1891		Brooklyn Eagle 11 Sept. 2/4: Rum-dumb, [...] an habitual soak.
1908		K. McGaffey Sorrows of a Show Girl Ch. v: Those Chicago rum-dums are the true sports, all right.
1927		D. Hammett ‘The Big Knockover’ Story Omnibus (1966) 292: Rumdum Smith, who killed lefty Read in Chi in 1916.
1931		D.M. Garrison ‘Oilfield Idyls’ in Botkin Folk-Say 146: I ain’t used to it like the rest of you rum-dummies is.
1938		D. Runyon ‘Dancing Dan’s Christmas’ in Runyon on Broadway (1954) 258: A guy [...] who is nothing but an old rum-dum.
1941		N. Davis ‘Don’t Give Your Right Name’ in Goulart (1967) 36: He’s a rum-dumb.
1949		N. Algren Man with the Golden Arm 25: Too late for roaches or old Skid Row rum-dums.
1951		E. Hemingway letter 4 Jan. in Baker Sel. Letters (1981) 717: I coudn’t be interested when he was a rum-dumb.
1953		T.A.G. Hungerford Riverslake 24: Oh, one of the rum-dums. Not a bad sort of a poor bastard, but the grog’s got him.
1956		N. Algren Walk on the Wild Side 33: Playing the fool for the same gang of cactus-headed rundums for whom he always played the fool.
1966		R. Stone Hall of Mirrors (1987) 63: Rum dums each and all!
1970		E. Thompson Garden of Sand (1981) 294: ‘Hey! Not it all!’ Bill pulled down the bottle. ‘Kids’s a rumdumb.’.
1974		(con. 1945) E. Thompson Tattoo (1977) 412: He could wind up some tapped-out old Asiatic rumdum, shuffling along the back streets of that godforsaken land.
1986		S. King It (1987) 89: Gresham Arnold, a rumdum of some local repute.
1999		J. Ellroy ‘My Life as a Creep’ in Destination: Morgue! (2004) 129: The Green Gables sucked. It was full of hypes and elderly rumdums.

2. an insane person.

1937

B. Dai Opium Addiction in Chicago 203: Rumdum. An insane person.

3. an inferior or worn out thing.

1962

R. Serling ‘The Whole Truth’ in New Stories from the Twilight Zone 8: That’s a heap! A rum-dum. It hasn’t got any rings. It hasn’t got any plugs [...] and it’ll eat up gasoline.