The tailbone sits at the very base of the spine, so dream books read it as your foundations — what you rest on when you sit still. A sore or broken one is linked to a sense that your footing has been knocked.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.