Your tailbone is the small pointed bone at the very bottom of your spine, right where you sit. Doctors call it the coccyx, but in ordinary talk almost everyone says tailbone — it is the plain, everyday word, and it is what you would say to a friend or even to a doctor. You meet it most often in stories about falling: you land hard on a step or a ski slope and you bruise your tailbone, and it hurts to sit down for weeks afterwards. The name is honest about what the bone is. It really is the leftover of a tail. It is made of a few tiny bones joined together at the end of the backbone, in the same place where a cat or a dog has a tail. Human babies have a small tail before they are born, and it disappears; this bone is what stays. Grammar-wise it is a normal countable noun, and you can write it as one word, tailbone, or as two, tail bone — one word is more usual now. Note that English says my tailbone, your tailbone, where Chinese often leaves the possessive out.

尾椎、尾骨（醫學上叫尾骨；台灣人講「摔到尾椎」最自然）