Meaning

noun wedge-shaped bone consisting of five fused vertebrae forming the posterior part of the pelvis; its base connects with the lowest lumbar vertebra and its tip with the coccyx

originFrom 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” —…

Etymology from Wiktionary, CC BY-SA 3.0