Holy Of Holies Meaning

(informal, humorous) One's private retreat, inner sanctum.

Example: The most sacred place within a sacred building.
1882. Franz von Reber, Joseph Thacher Clarke, History of Ancient Art, page 146:
  The holy of holies, a cubical space of ten cubits on the side, was separated from the larger antechamber by four columns, which were also covered with gold and stood upon silver sockets; they bore a second curtain of four colors.