The Collaborative International Dictionary of English v.0.48:

Sanctum \Sanc"tum\, n. [L., p. p. of sancire to consecrate.]
   A sacred place; hence, a place of retreat; a room reserved
   for personal use; as, an editor's sanctum.
   [1913 Webster]

   Sanctum sanctorum [L.], the Holy of Holies; the most holy
      place, as in the Jewish temple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sanctum sanctorum
    n 1: a place of inviolable privacy; "he withdrew to his sanctum
         sanctorum, where the children could never go" [syn:
         sanctum, sanctum sanctorum]
    2: (Judaism) sanctuary comprised of the innermost chamber of the
       Tabernacle in the temple of Solomon where the Ark of the
       Covenant was kept [syn: holy of holies, sanctum
       sanctorum]