What is Buffalo pen?

Buffalo pen is a colloquial term used to describe a prison or jail cell, especially one that is overcrowded and unsanitary. The term is thought to have originated in the United States, where buffalo were once commonly kept in pens or enclosures. The comparison of a prison cell to a buffalo pen is meant to evoke a sense of cramped, dirty, and unpleasant conditions. It is often used in informal speech or writing to express a negative attitude towards prison life.