The jaw is read as determination and the force of what you hold on to or say. A set jaw stands for stubbornness; a slack one, astonishment.
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.