The tribe folk (族民 Zoku-min) refers to the sky people living in the slums who are thought to be descendants of criminals.
The origin of the terminology is not known as the slum folks are not heard referring to themselves as a tribe. The city people and the slum folks apparently live together on the same floating landmass and it is not confirmed whether they are a group of people who migrated at some point of time before the story.
The people who live in the city condescendingly call the poor people from the slums as tribe folk or "filths" (汚れた者 Kegareta-sha). They regard the tribe folk as disgusting and does not want them crossing over the city. Most of the people caught red-handed and dropped into the Abyss are individuals originating from the slums, as a result people in the city only see those living in the place as descendants of criminals and see them as outsiders. Whenever a crime happens the people tend to believe it must be another tribe folk.
- Manga Chapter 1