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1. The innermost sanctuary of a temple, particularly the tabernacle in the Temple of Jerusalem, which housed the Ark of the Covenant. The Latin phrase translates to "holy of holies." The sanctum sanctorum is a very sacred place—it's where the Ark of the Covenant was kept.
2. By extension, any place that is regarded as very private and sacred. The basement is my sanctum sanctorum, and I'm trying to relax down here, so take your drama somewhere else.