Oubliette
“An oubliette (from the French oubliettes (noun plural)) was a form of dungeon which was accessible only from a hatch in a high ceiling.”
and also:
“To exit an oubliette was nearly impossible without outside help. The word comes from the same root as the French oublier, “to forget,” as it was used for those prisoners the captors wished to forget. Most prisoners were left to die of starvation."
