Green’s Dictionary of Slang

park n.1

[SE park, recreational area]

a prison.

[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 132: The Park is also the rules or privileged circuit round the king’s-bench or fleet . ‘The park is well stocked,’ when many prisoners have obtained the rules.
[UK]Halliwell Dict. Archaic and Provincial Words II 605/1: Park [...] Slang term for a prison. York.

SE in slang uses

In compounds

park ape (n.)

see under ape n.