/ ˈkɒksɪks; NAmE ˈkɑːk-/ noun
( pl. coc·cyxes or coc·cy·ges / ˈkɒksɪdʒiːz; NAmE ˈkɑːk-/) ( anatomy ) the small bone at the bottom of the spine
SYN tailbone —picture at skeleton
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WORD ORIGIN
late 16th cent.: via Latin from Greek kokkux cuckoo (because the shape of the human bone resembles the cuckoo's bill).