From Sahaja Yoga Encyclopedia

We have to know that our Creator is a very great, very understanding personality, is a Divine personality. And He has placed within us all the arrangements to get our self-realisation. Self-realisation is the connection with the all-pervading power of Divine Love. For that within us is placed, in the triangular bone called as sacrum, a power. All human beings have this power in the sacrum bone. Sacrum means sacred, and Greeks also knew that it was a sacred bone, so they called it sacrum. So it’s already there, and it has that power which is in three and a half coils. (HH Shri Mataji Nirmala Devi, Public Program in the Caffè della Versiliana, Versilia (Italy), August 31, 2001)

From Wikipedia

English sacrum was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the Late Latin name os sacrum "sacred bone", itself a translation of Greek ἱερόν ὀστέον, the term found in the writings of Galen. Prior to the adoption of sacrum, the bone was also called holy bone in English, paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein]) and Dutch heiligbeen.

How the Sacrum got its name

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.