For a long time, cities were considered safe havens compared to the roads on the countrysides where highwaymen and robbers preyed on travellers, but what are the earliest known examples of "bad neighbourhoods" as we think of them, riddled with poverty and crime?
Edit: I am aware of the origins of the word "ghetto", and I thought it was evident from my text that I didn't refer to Jewish quarters in medieval Europe specifically, unless they fit the description above.
The Subura of Rome had a bad reputation as early as the late Republican period. It had poor drainage so was a relatively low-cost-of-living area yet close to the city centre. It gained a reputation for being a red-light district. Julius Caesar, however, established a house there, possibly to enhance his reputation as a "man of the people".
The term 'ghetto' started in medieval Europe to describe the neighborhoods Jews were forced into. I do not know whether crime was unusually high there or not.
'Bad neighborhoods' as we know them can be traced back to the rapid urbanization that occurred in Europe during the Industrial Revolution. Crime was more prevalent in the low income areas that developed to house the working class.
It'd be great for someone to provide better sources. My answer is based off of knowledge gained from having a history major and from years of teaching middle-high school history.