Green’s Dictionary of Slang

(UK prison) to lock up a cell for the night; thus unchubb, to unlock.

1950		P. Tempest Lag’s Lex. 45: chubb, to. To lock. to chubb a man in is to lock him in. ‘Unchubb’ = unlock.
1989		J. Morton Lowspeak 39: Chubb – to lock.