Greek to me

That's Greek to me or It's (all) Greek to me is an idiom in English, expressing that something is not understandable.

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".

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:

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:

...
Wikipedia