1570s, "holy place of the Jewish tabernacle," from Latin sanctum "a holy place," as in Late Latin sanctum sanctorum "holy of holies" (translating Greek to hagion ton hagiou, translating Hebrew qodesh haqqodashim), from neuter of sanctus "holy" (see saint (n.)). In English, sanctum sanctorum attested from c. 1400; in sense of "a person's private retreat" from 1706.
