In The House of Mirth, what are oubliettes?

Oubliette is a French word that means a hidden dungeon where the only opening is in the ceiling.

In House of Mirth, Lily Bart considers her approach to meeting new people:

Training and experience had taught her to be hospitable to newcomers, since the most unpromising might be useful later on, and there were plenty of available oubliettes to swallow them if they were not.