sacrum

noun

Etymology: From Latin os sacrum (“holy bone”), a calque of Ancient Greek ἱερὸν ὀστέον (hieròn ostéon). Apparently so called either because the sacrum was the part of the animal offered in sacrifice or because of a putative belief that it is where a person's soul resides. A third explanation is that the term is a translation of Ancient Greek ἱερόν (hierón), which has two meanings: “holy, sacred”, and “big” — big being a more appropriate description of the sacrum — but compare.

1. A large triangular bone at the base of the spine, located between the two ilia (wings of the pelvis) and formed from vertebrae that fuse in adulthood.