the.com/holy of holies

means The most sacred, innermost, off-limits part of something — originally the inner sanctum of the Temple where God's presence was believed to dwell.

from From Hebrew 'qodesh haqqodashim,' literally 'holy of the holies' — Hebrew's way of making a superlative by stacking the noun on itself (like 'king of kings'), meaning the holiest spot in all holiness. It named the innermost chamber of the Tabernacle and later the Temple, sealed off by a veil, entered only by the high priest, only on Yom Kippur. The phrase passed into English through Bible translation and kept its weight, now used for anything treated as untouchably sacred — a lab, an archive, a locked door.