1577, from L. sanctum "a holy place," as in L.L. sanctum sanctorum "holy of holies" (from Gk. to hagion ton hagion, from Heb. qodesh haqqodashim), from neut. of sanctus "holy" (see saint). In Eng., sanctum sanctorum attested from c.1400; sense of "a person's private retreat" is from 1706.