Possible Clues:

• Terminal triangular bone of the vertebral column
• Remnant of a vestigial tail
• Human tail?
• Bone below the sacrum
• Bone at base of the spine
• End of the human spinal column
• Bony thing, angle on figure by the sound of it?
• A human 'tail'
• Bone picked up from the last of half a dozen roosters?
• Tail vertebrae