Green’s Dictionary of Slang

bum n.2

also bumb, bum-bailey, bum-bailiff
[? SE bound, bailiff (Blackstone, Commentaries, 1768) or physical proximity of the bailiff to those being arrested (Hotten, 1867)]

[mid-17C–1900s] a bailiff.

In compounds

bum trap (n.) [trap n.1 (3)]

[mid-18C–mid-19C] a bailiff or bailiff’s assistant.