Green’s Dictionary of Slang

a mixture of rum, water and sugar.

1751		Smollett Peregrine Pickle (1964) 6: He and my good master Hatchway come hither every evening, and drink a couple of canns of rumbo apiece.
1765		Caledonian Mercury 14 Oct. 1/2: Hadst thou ever the happiness to be in a party of choice spirits over a sneaker of rumbo?
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1821		W. Scott Pirate (1893) 418: Regaling themselves with a can of rumbo .
1843		W.L. Rede Our Village II ii: Scud and make a bowl of rumbo!
1886		Sporting Times 10 Apr. 3/3: I orders a toothful of rumbo.
1889		A. Conan Doyle Micah Clarke 106: He found a hogshead of rumbo which was thrown up from a wreck.