1911 Encyclopædia Britannica/Oubliette
OUBLIETTE, a French architectural term (from oublier, to forget), used in two senses of a dungeon or cell in a prison or castle which could only be reached by a trap-door from another dungeon, and of a concealed opening or passage leading from a dungeon to the moat or river, into which bodies of prisoners who were to be secretly disposed of might be dropped. Viollet le Duc (Dict. de l’architecture) gives a diagram of such an oubliette at the castle of Pierrefonds, France. Many so-called “oubliettes” in medieval castles were probably outlets for the disposal of drainage, refuse, &c., which at times may have served for the getting rid of prisoners.