Green’s Dictionary of Slang

1. (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.

2. (N.Z.) the nightly locking of public lavatories by the authorities.

2007

W. Ings ‘A Convenient Exchange’ in Public Space 3 17: n. Chubbing, described the penchant local councils developed for chaining and locking certain bogs at night.