Open mouth
A mouth held open — speech about to come, or speech stopped halfway, or the body left undefended at the front.
An open mouth in a dream is the dream's image of a front held open — possibly to speak, possibly arrested mid-speech, possibly because the closing reflex has lapsed. The mouth open is undefended; the inside is visible. The dream brings the open mouth when something in your life — a public statement about to be made, a confession arrested halfway, a moment of shock — has left your front line open and is being read by the room. A mouth open to say a clear sentence is the dream affirming that the voice has been committed. A mouth open with nothing coming out is the dream noting that the speech was begun and could not be finished, and the opening itself is now the expression.