That's Greek to me or It's (all) Greek to me is an idiom in English, expressing that something is not understandable.
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The idiom is typically used with respect to the foreign nature, complexity or imprecision of verbal or written expression or diagram, often containing excessive use of jargon, dialect, mathematics, science, symbols, or diagrams. The metaphor makes reference to Greek (either ancient or modern), as an archetypal foreign form of communication both written and spoken. Technically, the phrase is classified as a dead metaphor, meaning that its components cannot be used separately, unlike for example the dormant metaphor "foot of the hill", which permits saying "that hill has a tree at its foot".
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"). This phrase was increasingly used by monk scribes in the Middle Ages, as knowledge of the Greek alphabet and language was dwindling among those who were copying manuscripts in monastic libraries.
The usage of the metaphor in English traces back to early modern times, and it is used 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.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 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.The expression is almost exclusively used with reference to the speaker (generally "Greek to me"); Dekker's "Greek to him" is rare.
Variations
Different languages have similar formulations. Many have picked the point of reference to be a foreign language with another alphabet or writing system.
This is an example of the usage of demonyms in relation to the ability of a people to be understood, comparable to the development of the words barbarian (one who babbles), Nemec (Slavic for "the mute one" indicating Germans).
In other languages
In an article published by Arnold L. Rosenberg in the language journal Lingvisticæ Investigationes, he claimed that there was a popular "consensus" that Chinese was the "hardest" language, since various non-English languages most frequently used the Chinese language in their equivalent expression to the English idiom "it's all Greek to me". Also, David Moser of the University of Michigan Center for Chinese Studies made the same claim as Arnold L. Rosenberg.