Green’s Dictionary of Slang

(UK prison) the initiatory welcoming of a new prisoner, paid for by a mandatory fee of 2s 6d.

1843		Northern Star 23 Dec. 7/3: When a poor prisoner is brought in, he is called upon to pay two shillings and sixpence for what is termed ‘chumming up’ and if he is unable to pay that sum the coat is taken off his back and sent to pledge to make the amount, which is expended in ale.
1844		Bradford Obs. 18 Jan. 6/5: Mr Miller: Be good enough [...] to describe the ceremony of ‘chumming up’. Boot: When a new prisoner comes in he is welcomed by the prisoners who are in the prison and beat round with the chumming instruments [...] Mr Miller: And after this ceremony [...] do the prisoners demand from their new brother-prisoner any money? Boot: Yes. They half-a-crown from him.
1859		Hotten Dict. of Modern Sl. etc. 21: CHUMMING-UP, an old custom amongst prisoners when a fresh culprit is admitted to their number, consisting of a noisy welcome, — rough music with pokers, tongs, sticks and saucepans. For this ovation the initiated prisoner has to pay [...] half a crown, — or submit to a loss of coat and waistcoat.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].