Green’s Dictionary of Slang

1. (US) a political ‘slush’ fund.

1884		Boston Jrnl (MA) 1 Nov. 1: We are accustomed to barrel-campaigns here... the Democrats depend upon carrying it with money [F&H].
1888		Florida Times Union 11 Feb. 4: Mr. Flower was the nominal candidate of the anti-Cleveland men four years ago, and with the aid of his barrel they achieved some show of success [F&H].
1891		J.A. Riis How the Other Half Lives 89: About election time, especially in Presidential elections, the lodging-houses come out strong on the side of the political boss who has the biggest ‘barrel’.
1913		R.M. LaFollette Autobiog. 198: The nominations of the party will not be the result of ‘compromise’ or impulse, or evil design — ‘the barrel’ and the machine [DA].
1926		L.C. Wimberly ‘Amer. Political Cant’ in AS II:3 135: An ‘angel’ [...] is a man of wealth who makes—with no unquestionable motives—heavy contributions from his ‘bar’l’ towards the expense of an election.

2. (Aus. prison) a benefit night, with the proceeds dedicated to the payment of defence counsel or the arrested criminal .

[1954		Murrumbidgee Irrigator (Leeton, NSW) 13 Apr. 3: The Sydney Sub-branch [of the Old Yanconians] is holding a ‘Barbecue and Barrel Night’ [...] to enable country chaps down for the Easter show to be able to meet their old school mates.
1972		Parramatta Jail Gloss. B. Moore (ed.) (2023) 53: barrel benefit night, proceeds going towards payment of bail or defence counsel of crim arrested.
2001		Prison & Drug Sl.: NSW Corrective Services n.p.: Barrel, a, a party (usually on Sunday) with kegs containing beer.