Green’s Dictionary of Slang

Horse, the n.

also Old Horse, the

1. [mid-19C] Horsemonger Lane prison, Southwark, London [erected 1799 as a model prison, it lasted until the 1880s; it was outside this prison on 13 November 1847 that Charles Dickens witnessed the public hanging of the murderers Frederick and Maria Manning].

2. [mid-19C] Bridge Street prison, Blackfriars.