Sheet anchor is an expression that refers to something that provides strength and support when everything else fails. It is often used in nautical contexts, where the sheet anchor is the last line of defense against the ship drifting away. There are several synonyms for sheet anchor such as lifesaver, mainstay, rock, backbone, bulwark, and cornerstone. Each of these words reflects the same idea of providing stability and security in difficult times. So, whether you're talking about a person, a plan, or an object, these words can be used to describe something that provides an anchor in uncertain times.