Why do people say "It's all Greek to me" when they don't understand something?

This expression comes from the first act of Julius Caesar, a play by William Shakespeare. It was uttered by Casca when he was describing the words he had heard Cicero use regarding Caesar's refusal to accept the crown of Emperor.

In fact, Cicero was speaking Greek to prevent an eavsdropper from understanding what he said; the expression caught on as a way of showing confusion about something said.