Os and Tail bone

Os noun - A hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known.

Tail bone and os are semantically related. In some cases you can replace phrase "Tail bone" with "Os", this terms are similar.

Tail bone noun - The end of the vertebral column in humans and tailless apes.

Os and tail bone are semantically related. You can use "Os" instead a noun phrase "Tail bone".