Greek to me

Origins

It may have been a direct translation of a similar phrase in Latin: Graecum est, non legitur ("it is Greek, [therefore] it cannot be read"). The phrase is widely believed to have its origins among medieval scribes. While most scribes were familiar with Latin, few people in medieval Western Europe, even among the intellectual classes, were schooled in Greek. When copying classic manuscripts they would frequently encounter passages and quotations in Greek which they would have no way of translating, and as such would note the phrase in the margins.

Recorded usage of the metaphor in English traces back to the early modern period. It appears in 1599 in Shakespeare's play Julius Caesar, as spoken by Servilius Casca to Cassius after a festival in which Caesar was offered a crown:

CASSIUS: Did Cicero say any thing?

CASCA: Ay, he spoke Greek.

CASSIUS: To what effect?

CASCA: Nay, an I tell you that, I'll ne'er look you i' the face again: but those that understood him smiled at one another and shook their heads; but, for mine own part, it was Greek to me. I could tell you more news too: Marullus and Flavius, for pulling scarfs off Caesar's images, are put to silence. Fare you well. There was more foolery yet, if I could remember it.

— William Shakespeare, The Tragedy of Julius Caesar (1599)

Here, Casca's literal ignorance of Greek is the source of the phrase, using its common meaning to play on the uncertainty among the conspirators about Cicero's attitude to Caesar's increasingly regal behaviour.

Shakespeare was not the only author of this period to use the expression. It was also used in 1603 by Thomas Dekker in his play Patient Grissel:

FAR: Asking for a Greek poet, to him he fails. I'll be sworn he knows not so much as one character of the tongue.
RIC: Why, then it's Greek to him.

— Thomas Dekker, Patient Grissel (1603)

The expression is almost exclusively used with reference to the speaker (generally "Greek to me"); Dekker's "Greek to him" is rare.