oubliette (n.)

"secret dungeon reached only via trapdoor and with an opening only at the top for admission of air," 1819 (Scott), from French oubliette (14c.), from Middle French oublier "to forget, show negligence," Old French oblier, oblider, from Vulgar Latin *oblitare, from Latin oblitus, past participle of oblivisci "to forget" (see oblivion). Used for persons condemned to perpetual imprisonment or to perish secretly.