holy of holies

1. Judaism. The sanctuary inside the tabernacle in the Temple of Jerusalem, in which the Ark of the Covenant was kept.
2. Eastern Orthodox Church. The bema or sanctuary in a church.
3. A place of awe.

[Translation of Late Latin sānctum sānctōrum (ultimately translation of Hebrew qōdeš haq-qodāšîm) : sānctum, neuter of sānctus, holy + sānctōrum, neuter genitive pl. of sānctus.]

A place of awe or sacredness, as in The corporate board room is the holy of holies here. This expression is a translation of the Hebrew term for the sanctuary inside the tabernacle of the Temple of Jerusalem, where the sacred Ark of the Covenant was kept (Exodus 26:34). Its figurative use dates from the second half of the 1800s.

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The noun has 2 meanings:

Meaning #1: (figurative) something regarded as sacred or inviolable

Meaning #2: (Judaism) sanctuary comprised of the innermost chamber of the Tabernacle in the Temple of Jerusalem where the Ark of the Covenant was kept
  Synonym: sanctum sanctorum