Visiting poor or run-down neighborhoods, usually out of curiosity rather than genuine concern, or doing something beneath one's usual standards.
From 'slum' (a poor, overcrowded area) which emerged in the 1800s, possibly from slum as a back-formation or from 'slug' in dialect. The -ing form suggests the trendy Victorian practice of wealthy people touring poor areas.
In Victorian England, wealthy people would actually go 'slumming' as entertainment, like visiting a theme park of real human suffering—it became such a phenomenon that it led to early urban reform movements just to end the gawking.
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