oubliette
Definitions
A secret dungeon with access only through a trapdoor in its ceiling.
A dungeon or pit in which prisoners were formerly confined and forgotten.
Examples
The prisoner was cast into the dark, windowless oubliette, where he was left to be forgotten by the world above.
Exploring the ruins of the ancient castle, the tourists were horrified to discover a narrow oubliette hidden beneath the floorboards of the dungeon.