It's all Greek to me!
The expression is a direct translation from Latin: “Graecum est; non legitur”
. It refers to the fact that Greek was used as a classic form of communication by some in the middle ages, but was not understood by the majority. The idiom appears in 1599 in Shakespeare's play Julius Caesar
. A similar expression also exists in English: “It's double Dutch.”, a reference to the difficulty of the Dutch language.