Green’s Dictionary of Slang

chum v.

[chum n. (1); verb use coined by John Wesley (1703–91) in 1730]

1. [mid-18C–1930s] to put someone in a position of sharing accommodation, esp. in a prison cell.

2. [mid-18C+] (also chum in, chum it, chum up) to live with, to befriend.

3. [mid-19C+] (also chum in) to join in with.

4. [2000s] to keep a lookout for someone.

In phrases

chum along with (v.) (also chum around with, with, ...up with)

1. [late 19C+] to become friendly with.

2. [1910s] (UK Und.) to work as an accomplice with.

chum with (sfx)

[late 19C] to accompany; to travel with.