Sacer, sacra, ____: a polar adj., not only holy but also wicked (6)
I believe the answer is:
sacrum
(Other definitions for sacrum that I've seen before include "Triangular bone that forms back of pelvis" , "Triangular bone forming back of pelvis" , "Wedge-shaped bone forming the back part of the pelvis" , "Bone in the lower back" .)