What was Big Willy Shakes going for?
This is one of those rare times when Shakespeare is as straightforward as he seems. This phrase really just refers to something foreign or indecipherable. Have you ever watched a movie with subtitles and the people on screen speak to one another for several minutes, and the English translation is just one short line?
You think, it can't be right. Did they forget the rest of the translation? Since you don't know the language, you have no way of knowing. That's exactly what Casca is saying here. It's all the same sound when it's a foreign language. He can't understand one word from any other, because he knows none of them.
Back in medieval times, the phrase went a little something like this: "It cannot be read, it is Greek." But Shakespeare gave it his own little twist and the rest was history. We all go around using Shakespeare's version nowadays.