A slop is a slum, a neighborhood with low quality housing. In some cities, like The Hague, the word slop was used for a particular little type of slum, a small alley in between main streets, lined with cheap housing.
During the 1800s, many cities grew dramatically. Factories offered jobs, and drew many people from the countryside to the cities. Housing could not keep up with demand, and every little bit of space was used for housing.

Slum in Breda, 1936. Credits: Stadsarchief Breda.