sanctum sanctorum
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Latin for 'Holy of Holies,' originally referring to the innermost and most sacred chamber of the Temple in Jerusalem; commonly used to mean a private or sacred place.
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Mentioned once in You Can't Tell A Crook By His Cover.
You Can't Tell A Crook By His Cover
Season 1 · Episode 15 · S01E15
“And we now finally come to the sanctum sanctorum: my studio.”