Money paid by the richer sort of prisoners in the Fleet and King’s Bench, to the poorer, for their share of a room. When prisons are very full, which is too often the case, particularly on the eve of an insolvent act, two or three persons are obliged to sleep in a room. A prisoner who can pay for being alone, chuses two poor chums, who for a stipulated price, called chummage, give up their share of the room, and sleep on the stairs, or, as the term is, ruff it.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Chum * ChunkNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes