That's Greek to me or it's (all) Greek to me is an idiom in English referring to material that the speaker finds difficult or impossible to understand. It is commonly used in reference to a complex or imprecise verbal or written expression, that may use unfamiliar jargon, dialect, or symbols. The metaphor refers to the Greek language, which is unfamiliar to most English speakers, and additionally uses a largely dissimilar alphabet.
It may have been a direct translation of a similar phrase in ("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:
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:
The expression is almost exclusively used with reference to the speaker (generally "Greek to me"); Dekker's "Greek to him" is rare.
Other languages have similar formulations, some referring to Greek but many referring to foreign languages such as Chinese or Spanish. Many refer to a language with a different 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 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". David Moser of the University of Michigan Center for Chinese Studies reached a similar conclusion.