The Latin phrase sanctum sanctorum is a translation of the Hebrew term Qṓḏeš HaQŏḏāšîm (Holy of Holies) which generally refers in Latin texts to the holiest place of the Tabernacle of the Israelites and later the Temples in Jerusalem, but also has some derivative use in application to imitations of the Tabernacle in church architecture.
The plural form sancta sanctorum is also used, arguably as a synecdoche, referring to the holy relics contained in the sanctuary. The Vulgate translation of the Bible uses sancta sanctorum for the Holy of Holies. Hence the derivative usage to denote the Sancta Sanctorum chapel in the complex of St. John Lateran, Rome.
The Latin word sanctum is the neuter form of the adjective "holy", and sanctorum its genitive plural. Thus the term sanctum sanctorum literally means "the holy [place/thing] of the holy [places/things]", replicating in Latin the Hebrew construction for the superlative, with the intended meaning "the most holy [place/thing]".
Use of the term in modern languages
Romance languages tend to use the form sancta sanctorum, treating it as masculine and singular. E.g., the Spanish dictionary of the Real Academia Española admits sanctasanctórum (without the space and with an accent) as a derivative Spanish noun denoting both the Holy of Holies in the Temple in Jerusalem, any secluded and mysterious place, and something that a person holds in the highest esteem.
German Catholic processions
Some regional branches of the Catholic Church, e. g. Germans, are wont to refer to the Blessed Sacrament, when adored in the tabernacle or in exposition or procession (e.g. on Corpus Christi), as the Holy of Holies. By custom, It is adored with genuflection; with a double genuflection, that is a short moment of kneeling on both knees, if in exposition; in the procession this ritual may be nonrigoristically alleviated, but at least a simple genuflection is appropriate when It is elevated by the priest for blessing or immediately after transsubstantiation. Personnel in uniform — which in Germany includes student corporations — give the military salute when passing by or in the moment of elevation.
The "enclosed house" of Hindu temple architecture
The garbhagriha in Hindu temple architecture (a shrine inside a temple complex where the main deity is installed in a separate building by itself inside the complex) has also been compared to a "sanctum sanctorum" in texts on Hindu temple architecture, though the Sanskrit term actually means "enclosed house" or "the deep interior of the house". However, some Indian English authors seem to have translated the Sanskrit term literally as "womb house". 
- 2 Chronicles 5:7, in Latin (Vulgate): "Et intulerunt sacerdotes arcam foederis Domini in locum suum, id est, ad oraculum templi, in Sancta sanctorum subter alas cherubim". In English (King James Version): "And the priests brought in the ark of the covenant of the Lord unto his place, to the oracle of the house, into the most holy place, even under the wings of the cherubim".
- Mountains of the God — Page 49 Kuldip Singh Gulia The Architecture of Temples Most of the architectural elements are the same in all temples and each has a specific name. The sanctum sanctorum is called the garbha griha — the womb house. The garbha griha is a square cell that is.