Green’s Dictionary of Slang

workshop n.

(US Und.) a prostitute’s room within a brothel.

[US](con. late 19C) H. Asbury Barbary Coast 255: The ‘workshop’ was just large enough for a three-quarter-size iron bed, a wash-stand [...] a large bottle of lysol or carbolic acid, and a small chest or trunk, in which the prostitute kept her street attire.