Where does the term john in prostitution come from?

The term "john" in prostitution is believed to originate from the common use of the name "John" as a generic term for a man, much like "Jane" for women. It has been suggested that the name became associated with clients of sex workers in the early 20th century. Additionally, some theories propose that it may stem from the term "John Doe," used to refer to an unidentified male. Overall, it reflects a historical tendency to use common names as placeholders in various contexts.