If a place is rummier, it is not very nice or clean.
Rummier can make people think of old, dirty, or not safe places.
Examples:
This room is rummier than the other one.
The old house is getting rummier every year.
The rummier the place, the more I want to leave.
I don't want to go in there. It looks rummier than a pig's pen!
The rummier the cave, the more I wanted to explore it.
The rummier the room, the more I wanted to clean it.