rump-bone
/ˈrʌmpbəʊn/Chiefly Zoology. Now rare
noun
Any of various bones of the rump or hindquarters, specifically (a) a bone at the base of the spine; the coccyx, sacrum, or (in birds) the pygostyle.
Origin
Early 17th century; earliest use found in Helkiah Crooke (1576–1648), physician and anatomist. From rump + bone.