Definitions for oublietteˌu bliˈɛt
Random House Webster's College Dictionary
a secret dungeon with an opening only in the ceiling.
Origin of oubliette:
1810–20; < F, MF, <oubli(er) to forget
a dungeon with the only entrance or exit being a trap door in the ceiling
A dungeon only accessible by a trapdoor at the top.
Origin: From oubliette, from oublier.
The Nuttall Encyclopedia
an underground cell, perfectly dark, in which prisoners were subjected to perpetual confinement, was so called as being a "place of forgetfulness," or where one is forgotten; they were often put secretly to death.