Greek to me

That's Greek to me or it's (all) Greek to me is an idiom in English referring to an expression that is difficult to understand for the sayer. It is commonly a complex or imprecise verbal or written expression or diagram, often containing excessive use of jargon, dialect, mathematics, science, or symbols.
Although to some it may seem an insult, the metaphor simply refers to the Greek language (either ancient or modern) as typifying foreign communication, written or spoken, and may be a dead metaphor, one that has lost its original understood meaning.
Origins.
It may have been a direct translation of a similar phrase in ("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.
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 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.
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.