Green’s Dictionary of Slang

moby (dick) n.

[rhy. sl.; ult. the US novel Moby-Dick (1850) by Herman Melville]

1. prison [= nick n.3 (2a)].

[UK]M. Coles Bible in Cockney 28: He had Joseph chucked in the Moby.

2. the penis [= prick n. (1)].

[US]Maledicta IV:2 (Winter) 188: The old man can mean the Moby Dick itself.