in discripition

12. The phrase "It's all Greek to me" has become a common saying referring to something incomprehensible or meaningless. This saying comes from Casca's line: "...but for my own part, it was Greek to me," which originates from the Medieval Latin proverb Graecum est; non potest legi, meaning "It is Greek; it cannot be read." Explain how Casca's line is ironic.