coccyx

Concise Oxford English Dictionary © 2008 Oxford University Press:
coccyx /ˈkɒksɪks/

▶noun (pl. coccyges /-ɪdʒiːz/ or coccyxes) a small triangular bone at the base of the spinal column in humans and some apes, formed of fused vestigial vertebrae.

– derivatives
coccygeal /kɒkˈsɪdʒɪəl/ adjective.

– origin C16: via L. from Gk kokkux ‘cuckoo’ (because the shape of the human bone resembles the cuckoo's bill).