Great Caesar's Ghost!
This is an infrequently-used phrase describing surprise, and is mostly used by old people. Used among phrases such as "Dagnabbit!", and "Consarnit!".
But the mystery here, is the origin of the phrase. Was there actually a ghost named Caesar? Or was the phrase coined by some old coot who said whatever he god damned please? We must look at the circumstances.
If the phrase was made when someone had seen Caesar in ghost form, then we must wonder why he would yell the ghost's name, and wonder why the ghost was so great. After that, we must look at why only old people say this phrase. If you open your history book, you will see that Caesar was part of the Roman Empire a whole lot of years ago. Thus, if Caesar was really old, then his descendants would be just plain old.
From this evidence, we can surely assume that the term was coined by some old coot.