A dungeon, prison
The French had a word for it, and presumably a need. Medieval times were not the romantic era we imagine now, but were dangerous times, especially if you were an enemy of a powerful figure such as a member of the nobility or clergy. Many people were held, legally or illegally in castle donjons, whence we draw our word 'dungeon'. Some were held for torture, ransom, punishment or pleasure. Some were simply held because they were considered too dangerous to release.
Some were simply imprisoned to take them out of the way, permanently. These were frequently dropped into deep vertical cells, whose only opening was a trapdoor at the top. Here they might be left to rot and die, forgotten. This, then was the origin of both the cell and the word, drawn from the French 'oublier' (to forget).
The oubliette became a feature of many European castles. Some still exist, and the curious tourist may visit them at leisure. Warwick Castle, in the English West Midlands still has one, as does Amberley Castle in Sussex, but the grisliest must certainly be that at Leap Castle neat Kinnitty in Ireland. Here, the discovery of heaps of human bones on the floor of the bricked-up oubliette is a ghoulish reminder of the horrible end facing anyone thrown into it. This was a lonely, protracted death, certainly worse than being buried alive, as the prisoner would be able to hear people moving about him, ignoring his pleas for help and release.
The word has also been applied to the so-called priest holes, used for refuge and escape during the many periods of religious persecution which tore Europe apart from time to time. It is also used sometimes to describe any place where things may be sent to be forgotten - the most modern of which is the /dev/null device in Unix computing.