The Collaborative International Dictionary of English v.0.48:

iron maiden \i"ron maid"en\, n.
   An instrument of torture used in the middle ages, consisting
   of a box large enough to hold a person, often in the shape of
   a woman, and having multiple sharp spikes on the inside.
   Called also the iron maiden of Nuremberg.
   [PJC]

WordNet (r) 3.0 (2006):

iron maiden
    n 1: instrument of torture consisting of a hollow iron frame
         shaped like the human body and lined with spikes to impale
         the victim