The Collaborative International Dictionary of English v.0.48:

Oubliette \Ou`bli`ette"\, n. [F., fr. oublier to forget, fr.
   (assumed) LL. oblitare, L. oblivisci, p. p. oblitus.]
   A dungeon with an opening only at the top, found in some old
   castles and other strongholds, into which persons condemned
   to perpetual imprisonment, or to perish secretly, were
   thrust, or lured to fall.
   [1913 Webster]

         Sudden in the sun
         An oubliette winks. Where is he? Gone.   --Mrs.
                                                  Browning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oubliette
    n 1: a dungeon with the only entrance or exit being a trap door
         in the ceiling