Final answer:
The phrase in question is from the novel "The Satyricon" by Petronius. It refers to sexual activities and expresses shock and disbelief at this immoral behavior.
Step-by-step explanation:
The phrase "who loosened and let down this brutal law?" is from the novel "The Satyricon" written by Petronius. In this context, the phrase refers to sexual activities and the question is querying the origins of the immoral behavior. The passage describes a situation where the protagonist encounters a person who solicits sexual activities. The phrase expresses the narrator's shock and disbelief at the existence of such behavior.