1. To live poorly, or in below-average quality. Lifestyle that is below widely accepted standards, in terms of health, nutrition and hygeine, safety, and intelligence. Often linked with impoverished lifestyle or frugal living.
2. In stasis, or semi-nomadic state, surviving on bare minimum or below normal standards of living.
Some of the Boston MA WBCN radio guest DJs are known for slumming it in Roxbury.
In the movie industry, when a mid- to high-profile actor, actress, or (less frequently) director accepts a project considered far enough beneath them to raise questions about their integrity, they are considered to be slumming it.
There are a couple ways this can pop up. A highly regarded actor that usually appears in dramatic, "serious" works can appear in "low brow" entertainment (e.g. Dame Judi Dench in The Chronicles of Riddick, Marlon Brando in Superman), someone famous taking a role considered damaging to their public image (e.g. Patrick Stuart in Masterminds), or otherwise a highly talented individual accepting a role on a project that they should have recognized was doomed from the start (e.g. Laurence Olivier in Inchon). Money and/or contractual obligation are usually the reasons, but many seem to genuinely want the role.
Typically, an actor or actress that is slumming it will look hilariously out of place in the film. Whether it's because they're horribly miscast or because they turn out to be the only competent element of the work varies from case to case.
Abe Vigoda was really slumming it in Good Burger.