Oubliette
This is a combination of dungeon and torture device from the Middle Ages. It was a small room or shaft where someone was confined and left to die.
It seems slightly different from a dungeon, though the words are often conflated (indeed, the Wikipedia page even redirects). Whereas a dungeon was a general prison, an oubliette was specifically designed for people to simply waste away until they died.
It’s from the French word for “forget.”
Why I Looked It Up
I was watching the 1984 movie Labyrinth, and it was used by several characters to describe a hole where the main character fell into.
Postscript
Added on
I ran across this: The Oubliette: A Torture of Unspeakable Horror
An oubliette was a specialized type of dungeon, with the only entrance a trap door at the top, agonizingly out of reach of the prisoner.
Often this horrible prison was shaped like a very narrow passage, not wide enough for the prisoner to sit down or even get down on his knees. He was forced to stand or lie prone as he starved to death. He could tilt his head back to see the grate, far above his head and out of reach, but that was all.
It includes a poem about the oubliette and a couple of videos of oubliettes that have been discovered in castle ruins.