Oubliette \oo-blee-ET\ , noun;
1. A secret dungeon with an opening only in the ceiling, as in certain old castles
It dates to 1777 from French oubliette, taken from oublier ("to forget"). Oublier is from Old French oblider, which is the Vulgar Latin derivative of Latin oblivisci ("forget").
Spooky, right? A dungeon with one way in a no way out. Just drop someone in the hole and forget about them. Yikes.