The os sacrum (sacred bone) was so named by the Romans as a direct translation from the older Greek hieron osteon. Explanations of the attribute "sacred" or "holy" in the past have included misinterpretation of the Greek word hieron, use of the bone in sacrificial rites, the role of the bone in protecting the genitalia (themselves considered sacred), and the necessity for the intactness of this bone as a nidus for resurrection at the Day of Judgment. A more plausible explanation may be that the holiness of the sacral bone was an attribute borrowed from the ancient Egyptians, who considered this bone sacred to Osiris, the god of resurrection and of agriculture.
Sugar O. How the Sacrum Got Its Name. JAMA. 1987;257(15):2061-2063. doi:10.1001/jama.1987.03390150077038