Hi, Karen. From the OED:

Lock, n. ...15. Originally: a hospital at Southwark in London, used for the isolation and treatment first of persons with leprosy and later of those with sexually transmitted diseases, esp. syphilis. In later use also: any hospital used for the treatment of persons with sexually transmitted diseases. In later use frequently attrib., esp. in lock hospital. Also with capital initial. Now hist.

OED gives quotes for "lock" only from as early as 1359.

The more explanatory "lock hospital" is found from 1766, but is presumably somewhat older.