graybar hotel

Referring to the gray metal bars of a cell. It was originally used to refer to the main jail facility in downtown Los Angeles, California; some inmates who were ashamed of admitting that they were in jail would tell friends and relatives who were out of town and that they were staying at the Graybar Hotel. Before it was torn down it was so well known that prisoners actually received mail addressed to the Graybar Hotel in Los Angeles.

来源：wiktionary