means The large, triangular bone at the base of your spine, formed from fused vertebrae, that wedges between your two hip bones and anchors the pelvis.

from From Latin os sacrum, 'the sacred bone,' itself a translation of the Greek hieron osteon. Why 'sacred' is debated: one tradition holds the bone was offered in sacrifice or thought to guard something holy within; another suggests 'hieron' here simply meant 'large' or 'strong,' and the 'sacred' reading is a later gloss. So the name we use every day may rest on an ancient mistranslation — vivid either way.