Green’s Dictionary of Slang

cob n.3

[orig. dial. cob, a small roundish loaf]
(UK prison)

1. a punishment cell.

[UK]Barrère & Leland Dict. of Sl., Jargon and Cant.

2. prison bread.

[UK]P. Tempest Lag’s Lex. 48: cob. The regulation prison ‘loaf’ of about 8 oz. weight. ‘A cob and butter’ = a prison loaf and a margarine ration. Together these commodities are worth anything from one to three ‘roll-ups’ in the prisoners’ market.