/ ˈseɪkrəm; NAmE ; ˈsæk-/ noun
( pl. sacra / -krə; NAmE / or sac·rums ) ( anatomy ) a bone in the lower back, between the two hip bones of the pelvis
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WORD ORIGIN
mid 18th cent.: from Latin os sacrum , translation of Greek hieron osteon sacred bone (from the belief that the soul resides in it).